It is at lvl 50 cause a line has to be drawn somewhere... UR staff decided to draw that line there to limit the number of eligible people.
if they drew it at lvl 60... a bunch of 50s will ask why
if they drew it at lvl 40... a bunch of 30s will ask why
its there cause it has to go somewhere.
other factors will be taken into account im sure, but lvl 50 is one of the starting points