JNT planned to bring back Ian Chesterton for Mawdryn Undead but unfortunately William Russell was too busy at the time. This is why The Brigadier is a mathematics teacher in the serial.
Russell and Jacqueline Hill were also to have done a cameo at the end of The War Machine but it didn't work out.
Also, Jamie McCrimmon is paired with the Sixth Doctor in a new series of audio dramas for Big Finish that starts shortly.
In answer to the original question, why should the series always be looking backwards? If a new production team is brought on board, it should be their right to move the show forward and create the Ian Chestertons for the future generation, which is what RTD did. Only difference with him is his era was closely interconnected between 2 Doctors and 3 series, so there was a lot of give and take.
But if Moffat goes his entire run without Rose, or Ace, or Tegan appearing, I don't see the problem. He's too busy creating new monsters or expanding on characters he created like River Song. Yeah I know there's a Dalek or two in there as well, but some things are obligatory.
And anyway, if the OP wants to know why the Doctor doesn't usually reunite with past companions, watch School Reunion again and listen to what the Doctor says on this very subject.
Alex