He was very underrated. He was very good in all his roles.

Thanks, interesting insight. He seems to have been underated at the time. I just don't know why he didn't get the love he deserved. He was a fantastic actor.

Thanks for the information. Much obliged.

There's never really been a definitive answer, but it seems the makers of the films were not that high on Pleasence. A number of actors were asked to play 2000Blofield before him, but all had commitments which made them unavailable.
Producer Harry Saltzman hired Czech actor Jan Werich to play Blofeld originally. Werich was the personification of sophisticated intellectual humor in Czech theater and film, and was known as the "Wise Clown". Casting a clown as a demonic villain backfired during production: It was felt that Werich looked too much like Father Christmas to be menacing and he was released after a week's shooting.
So anyways, Pleasence was a last minute replacement, and when the next film came along Telly Savalas, who was a hot commodity after
The Dirty Dozen
, was given the role. The very masculine Savalas was likely picked because reviews of Pleasence at the time generally felt that he lacked the physical presence needed for Blofeld.
Oddly enough, time has been very kind to his performance and he was even strangely helped by the "Dr. Evil" parody.