I hate the current production crew, fuckers, no respect for the 26 of the 34 broadcasting years of Doctor Who. So none of the '20th century' doctors were even approached, that means that they never planned to include them at all. The 50th only comes once, and its wasted on fans who also like the classic series.
Even some of the specials like BBCA's The companions di not feature any companions before 2005, they only talked about Sarah Jane because she was in the new show also.
They honestly may not have a choice in the matter. Auntie Beeb may be telling them "This is all we're giving you budget-wise. You want all these old actors, they'd better be willing to work for free. Otherwise, sod 'em."
Honestly, you think Paramount treats Star Trek
with disrespect sometimes...