the problem is that people just don't understand that level doesn't mean anything unless the pokemon evolves. With the Join Avenue and Wings introduced in gen 5, you can easily EV train any level pokemon (including level 100). UT is not as important as it used to be, not that having under-leveled legendaries was ever all that beneficial thanks to the move reminders. I believe an expiration time on GTS trades could solve many problems by clearing out pokemon from games that have been restarted and also removing unreasonable trades, both plausible to complete and not. Alternatively, searching by requested pokemon (only showing pokemon already seen) would make trading efficient as well (I don't claim this to be my own unique idea as it is mainly common sense, implementation difficulties aside).
As far as explanations go, I see everyone excusing the lvl 9-and-under legend requests as (presumably) failed of clone attempts, but I don't thing that accounts for this proliferation. This may have been an initial reason, but I have a feeling that a lot of it is new players thinking "Low level legend? That sounds cool! I want one too!" and adding to the problem.