Why 90 and not 120?
Well, one reason is that launching all these planes would take about an hour, and their recovery another hour (there are several catapults to launch airplanes, but it takes lots of time to put them on these catapults, while during landing it's less to do - plane lands, folds wings and is quickly taken to the side - but they can land only one at a time). Current US Navy standards talk about pace of one aircraft per 30 seconds for both these operations, but some delays always need to be taken into account.
And remember that after taking off they need to get into formation (and when the last planes will join, those that took off first were already burning their precious fuel for an hour). Granted that You'll leave some planes for self-defence, and that some fuel on the "first" planes can be refuelled from aerial tankers. But with smaller CV with smaller air wing You'd leave some planes for defence too, and tankers are few and their capability limited (not mentioning that such refuelling operation would also take time, during which fuel will be also burned...
).
There's also an issue of handling these aircraft on the carrier itself. That's not a fixed airfield, where each aircraft has it's parking space with infrastructure etc. On an aircraft carrier planes are continously moved back and forth and when You want to move a plane from point A to point B, often You have to move some other plane from point C to point D to make place for the first plane. With 120 aircraft it's insanity in action.
Btw. Nimitz class can theoretically carry even 120-130 aircraft (somewhat squeezed), but todays Carrier Air Wing is merely half of it.