Especially on lowcost airlines... People who would otherwise travel on busses and trains (have you seen the state of many of those?) now travel on aircraft.
For decades bus and train operators have been battling people putting knives to their seats, spraying grafitti in traincars and busses, chewing gum on seatbacks, etc. and all that's in full view of the other passengers.
In a toilet there's noone to see what you're doing, anonimity assured. Remember the public outcry when airlines were proposing installing TV cameras in toilets to catch terrorists using them to assemble bombs and weapons?
I wish I were flying