Imagine all the Bitcoin miners out there right now using their ASICs to do extremely efficient hashing in the hopes of generating a block reward. Let's didactically suppose there are 100 such miners total.

Now imagine Dinkycoin comes along and releases their cryptocurrency that uses the same hashing mechanism for the block reward. Initially they have the block difficulty level pretty low as there aren't that many people participating in it yet.

Dinkycoin hopes that eventually enough people will join the network so that the total hashing power will be something approaching Bitcoin's. Great. But atm their total hashing power is a very small fraction of Bitcoin's total hashing power. I don't care enough to go find out how infinitesimal it would be-- enough to say very small.

Now suppose one of the Bitcoin ASIC miners gets bored one day and decides they don't like Dinkycoin. If they devote their ASIC to mining Dinkycoin they will immediately have more hashing power that the rest of the Dinkycoin network. That would virtually guarantee they solve every block of Dinkycoin and reap every reward. They'd easily have 100% control of the network at that point.

For example-- suppose Dinkycoin says you need 20 confirmations before accepting that a transaction "went through." Well, the Bitcoin miner can use their enormous hashing power to go back 20/30/whatever blocks and build a competing branch from that historical point. And since the miner has 1000% more hashing power (or whatever the number is) than the entire Dinkycoin network, they can build a competing branch from that historical point that will eventually have solved more/more difficult block rewards than the branch the rest of the network builds off the current branch. Once the Bitcoin miner has a competing branch with greater total difficulty than the rest of the network, the network must accept that branch as the winner. Well, the devs can decide to do otherwise for whatever professed reason and roll back/hard fork/whatever. But that kind of "do over" would degrade trust in the cryptocurrency itself. Users would be afraid of another future attack. After all, they were initially told 20 confirmations was enough. How do they know 40 confirmations is any safer? (And again, if the attacker has superior hashing power to the rest of the network it isn't meaningfully safer.)

Of course such a miner would be wasting all of their hashing power to screw with Dinkycoin's network. But to screw up most PoW cryptocurrencies you only really need 51% of the total hashing power. Even with that the attacker will eventually always beat the rest of the network to produce the "longest" chain. (Also, I think you can still cause problems for many cryptocurrencies with even less than 51% control.) Anyhow, do the math with the real numbers and you'll see that a real Bitcoin miner could devote a tiny percentage of their mining power and still own fledgling cryptocurrencies like the Dinkycoin example.

Moreover, they can trivially collude with the other 99 Bitcoin miners to devote an even smaller part of their hashing power to owning Dinkycoin, or even automate the process of smashing any coin that uses their same hashing scheme, based on any criteria whatsoever.

And it gets worse because in the 51% percent attacks I've read about in the real world, it isn't a big time miner with a grudge. Rather, it's someone who saw a weak point in a combination of a dinkycoin, an exchange, and several other moving parts and just rented a hashing rig to pull off an attack. Most PoW cryptocurrencies naively assume miners will choose greed over destructive tendencies without ever investigating whether those two paths are really mutually exclusive.

Edit: clarification

Edit: what happened to the web site that compares hashing power of the various SHA256-based coins, scrypt, etc.? Those orders-of-magnitude differences are quite instructive. But I can't find a site that lays it out nicely.