Could be several things:

1) Group finder is pretty broken right now. Most of the time it doesn't work as intended.

2) If you are queuing as a tank or healer on your level 21 they find groups easier than a dps character.

3) It might just be that more people are using the group finder with characters in the 20ish range. Sometimes if you fall ahead or below the leveling curve (even if that is the curve for people leveling alts), you have a harder time finding groups.

4) Is your level 10 in a different faction than the level 21? Some alliances have fewer players in general, and since you can only group with people of your faction that could play a role.

Or any number of other factors could be at play...