Given that Bitcoin has become quite liquid, there might also be ways to short it. But yes, governments doing it for political reasons would seem like the most obvious incentive.

> What is the incentive for a 51% attack (on Bitcoin specifically)

Besides trying to destroy bitcoin, couldn't such an attack be used to simply drive up transaction fees for anyone who wanted to use it? For instance, if they refuse to process transactions with a fee below a threshold, they'd leave money on the table in the sort term, but eventually the higher fee requirement would have to be discovered and transactions reissued to pay it.