I think it was because he was so similar to Jon Pertwee's Doctor in many ways. They both had a sort of sartorial elegance, and if anything, the Master was more polite and affable (most of the time, anyway) in that incarnation than Pertwee's Doctor. He even had a sense of humour (in The Sea Devils, he playfully calls the Clangers an intriguing alien lifeform, although the joke is spoiled when his ally tells him that the Clangers are puppets).
BTW, it's Delgado. Sorry, Tony, I am a pedant.
In addition, Delgado played Hugh Conrad, a journalist, in Quatermass II. The scene where he gets taken over is a bit melodramatic, but well done, and it's strange to see this guy who played an excellent villain play a more normal role.