The short answer is that miners are hoarders, and this is based on a few things:

1. In theory difficulty and hash rate should follow the bitcoin price, but they don't. You'd think that as price decreases difficulty re-adjusts as miners pull out but difficulty and hash rate continue to increase and are completely disconnected from the price. Charts:

Difficulty:

https://blockchain.info/charts/difficulty

Hashrate:

https://blockchain.info/charts/hash-rate

Price:

https://blockchain.info/charts/market-price

Because of the disconnect, you can conclude that most mining business models are based on the future price of bitcoin. The miners are either true believers in the future potential, or they are effectively stuck because they have sunk upfront real USD but are currently mining at a loss and have no choice.

2. Most mining at the moment is being done at a loss. This continues from point 1, but if you look at a mining calculator your can work backwards and calculate at which electricity price point mining is currently profitable at.

As an input for our calculation, lets assume all miners are running the equivalent of a KnC Neptune, a new 20nm-based board that hasn't been released yet but it due out in a month or two (hah!):

https://bitcointalk.org/index.php?topic=347597.0

We will normalize their figures based on price per TH hashrate, and price per GW output. They promise 3TH for $10k, and they promise 2,300W.

So for each TH it is $3.3k upfront, and 733kw/h to power that TH.

Plug those figures into a calculator:

http://www.bitcoinx.com/profit/

Your gear is generating $32 per TH per day, so $96. To pay off only your hardware would take 104 days, this is with 0 electricity cost. You'd need to find electricity
at a few cents per kw to break even[0]. The sums aren't good.

[0] This is possible in only a couple of places, mostly scandinavian nations with geothermal electricity supply or nations where electricity is subsidized like China and India. Even at a low 8c/kw your hardware (which is being delivered) is paying itself off in 118 days at todays difficulty rates (which won't hold).

3. A big problem with mining is that your investment decision is inelastic (not sure if that is the right term). You decide to invest at todays difficulty and bitcoin rate but outside of cloud-based mining you don't actually get started mining after placing your order for at least a month, often 5-6 months. So that means when there are little openings in the window for mining profitability everybody piles on, and you find when you get up and running that your mining estimates are an order of magnitude off because a lot of other people had the same bright idea.

The hashrate and difficulty charts show that with their increases. If you plotted the announcements of new generation mining gear against those charts in the same way Google News does you'd likely find that each big spike is equal to in time as new_mining_gear_announcement_date + new_mining_grear_delivery_date

4. Newly mined coins can't be transferred for at least 100 confirmations. This was an impromptu patch applied by the bitcoin project and a number of markets to defeat some sort of bug, but I can't recall or can't seem to find the details at the moment and can't recall what it was fixing or if the 100 number is correct. If anybody knows the details of this, i'd be interested in finding it again.

So when looking at the blockchain for newly minted coins, you'll never see them move immediately, there would be an at least 100 transaction delay (which isn't long in real time terms).

5. On non-volatile days the markets combined trade around 15M coins. On highly volatile days like today the combined volume exceeds 120M coins. 3,600 new coins are mined per day, so their input ranges from 0.01% of coins traded to 0.002%.

Note that this is traded volume, rather than percentage of coins, and the low trading days are often only a few million transactions per day.

6. You can follow the newly mined coins on the Blockchain. Ghash.io is anywhere from 35-50% of the total hash rate (which in itself is controversial) and their newly generated coins go to this address:

https://blockchain.info/address/1CjPR7Z5ZSyWk6WtXvSFgkptmpoi...

You can see that most of the coins remain unspent, and GHash.io is a unique case where you'd expect more of the coins would be spent, which i'll explain in my next point.

7. The hash rate and return itself has now become commoditized and can be traded like a derivative. cex.io was a pioneer here and they have an active market for buying and selling GH/s mining. Their pool is the GHash.io pool mentioned above.

Their current price for a GH/s of mining hashrate is 0.01058690 BTC, or around $4.03 USD. If you compare to the yet unreleased Neptune above, it is $4030 per TH/s vs the Neptunes $3,333 per TH with no electricity costs - and you can move in/out as you wish.

I have previously profitably traded on cex.io using an automated trading bot. It hasn't been active in a long time because I haven't updated it to suit the new conditions.

Here is my referral link to signup to cex.io, I get 10% of whatever you purchase:

https://cex.io/r/0/nikcub/0/

Here is the base open source version of my bot, which doesn't have any of my strategies built in it, but which can be extended:

https://github.com/nikcub/cexbot

I'll likely bump the version currently there with my new strategy interface in the next few weeks since I believe there are new opportunities in GH trading again on CEX. If you're interested in trading strategies, get in touch. The most basic mechanism in that bot will take your mining return and pour it back into purchasing GH/s so that your returns are compounded.

A lot of the return you get in CEX is from the other coins that are mined alongside Bitcoin when you purchase GH/s.

Note that nothing in the bitcoin world really makes sense, so the rules from normal public stock markets or strategies that might work there don't apply in the bitcoin world. You can see from the disconnect between hashrate, difficulty and price that a large part of Bitcoin price and market movement is speculation and emotion driven. You can still take advantage of this, though - as a lot of the price is driven by news events and emotional reactions to them.

Since it is so easy to move in/out of mining on CEX, you'd assume their mined coins are more liquid than the miners who setup long-term projects and are looking for a return based on future bitcoin price, but even at the CEX.io address you see only a small number of coins being moved.

8. In theory it would be possible to go back through the blockchain and measure/quantify what percentage of newly mined coins in a certain time period are being spent. Anecdotally i'd estimate the percentage is very low, and that most coins in circulation are greater than a year old (eg. i'd estimate 99.5% of traded coins were mined +1 year ago).

tl;dr: the current mining business model is based on future bitcoin price, most newly mined coins are held.