Because that's what we say it should be
Honestly, I have no idea how a lot of us adopted the 120 second rule. I know the 10 sec/movie rule was put into place because there were a lot of people submitting 5 sec movies (which are way too short)
It might be because it's very easy to split movies up into clips that total 60 sec/page - 6 @ 10 sec, 5 @ 12 sec, 4 @ 15 sec, 3 @ 20 sec, 2 @ 30 sec.
I might have to look into that rule a bit more on Monday.