it's who is current. what so many people seem to forget is that every Lego set has to be accessible. This set is based on what's on TV right now, so kids who just got into the series will see the set and recognize those characters. It works the other way too. Kids who get the set will tune in on Saturdays and see Clara and 12, just like in the set. Any other figures would be really confusing. (also, I wouldn't be surprised if there was an extra head + hair for 11)
On the set, it looks amazing. Interestingly, it looks like 12 just regenerated (based on both his and Clara's outfits). The Daleks are awesome, the Angels are unexpected, and the TARDIS itself looks incredible! Both the exterior and the interior are FANTASTIC. I cannot wait to get my hands on this thing. I might even get two to get 12's outfit twice (for easy Matt Smith customing). I mean, look at that parts usage! the Exo-Force arms as railing, the stud with a hole in it as a Dalek plunger, the way the console's built. Not to mention the ingenious use of the Ninjago nunchuck piece as a Dalek eyestalk. Lego has truly outdone themselves.