There was a question which is now closed to answers. It was should past Doctor's return to the new series. It really should be titled (Why doesn't Who producer Julie Gardener want past Doctor's to return)

I'll give my reason.

Because returning doctors usually just mean a campy little reunion show devoted to in jokes rather than an actual program, with a story line. The equivalent of a clip show.

Old Doctor Who geek answer coming up:

The idea of regeneration first came about when the first Doctor, William Hartnell, decided to leave this hugely popular and successful show. He was rather old and wanted to do other things before he retired.

Because the show was massive the producers decided to introduce a nice little twist called "regeneration" which allowed the Doctor to change his appearance and character once his current body was worn out or dying. The trick worked very well and kept the show fresh. Indeed for about twenty-five years it was among the highest rated shows on British television.

To date there have been ten Doctors. Because I'm giving a complete answer, here they are:

William Hartnell
Patrick Troughton
Jon Pertwee
Tom Baker
Peter Davison
Colin Baker
Sylvester McCoy
Paul McGann
Christopher Eccleston
David Tennant

Sometimes previous Doctors have agreed to return to the role and there have been a few stories where Doctors have been "extracted" from the past to assist the current Doctor. These stories are:

The Three Doctors (1, 2 and 3)

The Five Doctors (1 (even though the actor was dead) , 2, 3, 4 (even though the actor refused to appear) ,and 5)

The Two Doctors (2 and 6)

There may be another one - but Im sure somebody else will fill in the blanks.

After Doctor Who fell out of fashion in the eighties it continued as a series of audio dramas (still going on today) featuring Doctors 5, 6, 7 and 8. These audio shows have also featured a couple of double-Doctor shows. Check out www.bigfinish.com.

To answer the question - finally - the reason that past Doctors do not return to the new show is that the show is aimed mainly at an audience who have no knowledge of the previous forty years. To involve too much back story would alienate them and reduce the show to a cult of die-hards. Since that would be commercial suicide they are largely ignoring the past and just using fresh ideas - and the best monsters.

What back story there is tends to be one-liner in-jokes for us old fans without breaking the pace of show.

Damn good idea if you ask me.

Hope this helps.