It appears to be because according to the people making the programs young people won't watch tv unless they find the leads attractive and obviously old people can't be attractive! The whole thing is totally crazy, but appears to be why the doctor is getting younger and younger, as well as why all the companions are young (and at least one is female)
It seems to be part of the whole idea that all tv (films etc) has to appeal to teenagers and 20s people - so why everything has to have sex and violence and lack of intellectual depth. Except things that are put into different niches - but that mostly seems to be not-fiction stuff.
(sorry for ranting - tv annoys me so much)