Wasn’t it a meme that Time Lords only had 12 regenerations? The Master got around that by taking over someone else’s body, but that was evil, and the Doctor wouldn’t do that. So he could only come back as one of his earlier regenerations. Perhaps the 12th Doctor was supposed to be a recycling of the 1st Doctor; they looked alike.
So any actor selected to portray the Doctor would have to resemble one of the first 11.
He’s already on his 13th. John Hurt, the War Doctor, is not among the 12.
Matt Smith was already on the 12th regeneration, but the High Council of Time Lords gave him more regenerations on Christmas.