For one thing, in the old days, far fewer people needed to be credited. Over the years, the unions have negotiated for more and more people that work on films to be credited, to the point where now the guy that sweeps the floor gets a mention. This has made credits much longer, to the point where nobody would put up with it all before the picture starts. By putting the bulk of the credits at the end, it saves time for the theater, because this is time that people are walking out anyway, so it doesn’t reduce the number of showings they can squeeze into a day.
One of the things my and my GF have in common is we wait till the credits are finished to leave. Sometimes we get rewarded, like at the end of the Iron Man credits. 