Because the basic model of shorting Bitcoin involves waiting for the market to go down, rather than up — which is the movement that most traders are looking for. This means that shorting can only be successful when the market is about to fall, rather than when it has finished falling.
This comes down to getting the predicting and the reading of the market right and sensing when factors are present that can lead to drops in the price. Of course, there are also very different ways in which the market can fall, in the short, medium, and long term.
You should look to short sell when you think that the market is ready to drop. But, this can be done in a few ways — traders can start to read external factors and news that are known to have a negative impact on the market. They can also utilize technical analysis which is intended to help traders read the market and predict the next move — both up and down.
The first time to start shorting the market is at the first sign of negative news brewing. Bitcoin is a speculators market and is susceptible to negative news, so it is worth shorting the market when some of the beginnings of bad news begin to surface. Bitcoin also moves fast so when things start turning, it is advisable to be quick with your short.
Another more efficient and sure fire way to get your shorting time right is to look at technical analysis which is intended to help traders predict moments and momentum, as well as breakouts and fall backs.
If the technical analysis charts point to momentum taking a turn towards a fall, it is a good time to get your shorts in even before there is a drop which allows for maximum potential for earnings from a fall.