Mainly it's that at 10NL people haven't learned to fold yet, and you make most of your money just value betting them to death and not really bluffing much. At 50NL most players have already learned that folding a lot is superior to calling a lot, so you make a lot of your money from stealing pots.

When I moved up from 10NL to 50NL (I actually skipped 25NL) in 08 I went from playing 13/4 to playing 25/18 @FR with a huge AF. That was me overadjusting and generally not knowing much about poker... but you get the picture.