The Cushing movies made a lot of changes to the basic storyline so it was easier to tell the two versions apart. Instead of being called the Doctor, Cushings version was explicitly named Doctor Who. He was an earthling and not a timelord. The Tardis was his invention rather than being stolen and had aradically different console room. Both Barbara and Susan were his grand daughters.
Regardless, One of the big problems with a separate movie would be that Doctor Who is far bigger than it was back then. When the Cushing movies were made, there was still only one Doctor so it was easier to reconcile the change as it being a retelling. Now we have had 11 doctors. A new doctor would likely cause some to believe that this is a new incarnation.
There has been attempts to reconcile the movies with continuity. One was that Cushings movies exist in contiunity as movies and were based on reports of the Doctors adventures but the facts were changed because few people knew of the Doctor's true nature. Another was that the movies were based ona book written by Barbara but that she changed some facts to protect the Doctor.