Any BTS 'lost' on any exchange (be it through a hack, fractional reserves or whatever) doesn't only hurt the depositor.
Imagine if loads of BTS went 'missing'... Oh deary me, what a catastrophe!
In the case of a theft, arguments would ensue over rollbacks and all that shit. Very bad problems. Much debate. Much harm.
If it becomes general knowledge that a large percentage of a cryptocurrency is in the hands of 'bad guys', it really puts people off.
Very bad P.R. problems follow. People don't want to get involved with something if they know 'baddies' own 10% of the market cap, do they?
We don't want to have to deal with something like this... but that's obvious.