So - it's like asking why Willie Sutton robbed banks.
(Plus - if you were a violent, drunken barbarian, already having the time of your life raping & looting - then what could possibly make things even better? Yes! - a giant fire!!)
Or - any time you've got a nasty cultural conflict, and libraries are near-and-dear to just one side - yes, obviously the other side will go after 'em. Ask an old Playground Monitor whether verbal bullies shuffle through various subjects & words, searching for whatever will get the worst emotional reaction from their victim.
(However much it mashes some people's buttons, I suspect that going after libraries for very calculated, 1984-ish reasons is relatively rare. If your side has effective control of the police and media, then maybe 0.01% of the population really cares that some dusty old book proves your current propaganda to be a lie.)
This is a popular saying but it is not really true. In an open and free society like western societies (at least some still) are, history is the domain of historians and not the state.
For example how could the plight of native Americans be anyone’s concern if the state controlled all history?
So yes, the victors like to rewrite history but we should be conscious of the fact that most of the history we read is actually pretty accurate and well founded, not simple propaganda.
Of course that might well be about to change in the US.