I really can't answer for the Mods, but *I* always thought it was because this board gets a lot of interest. We have over 6100 members. Lots of people sign up, post a few times, then they're off to other shiny places, say 10% for example purposes. If all of those 6100 could upload their own avatar from the word go... even 10% of that is an AWFUL LOT of server space wasted on people who are never heard from again.
By the time a person has made 100 posts, they're sufficiently addicted to earn a little space on the server.
But of course, that's just my thoughts and may bear no resemblence to reality (as usual)