When William Hartnell agreed to play the Doctor in the early '60s, the concept of regeneration didn't even exist. The creators of the show just made a program about a travelling eccentric - nobody involved thought that Doctor Who would spawn a fifty year legacy that would require it to switch out the lead actor every couple of years.
But during Hartnell's run, several problems arose. The actor suffered from arteriosclerosis (the hardening of artery walls, which slowly restricts blood flow) and his poor health made it difficult for him to keep up with the demands of filming. Plus, when producer Verity Lambert left (who was instrumental in getting Doctor Who off the ground), Hartnell struggled to get along with the new production team.
These factors resulted in Hartnell being forced to abandon the iconic character. Then, in a stroke of genius, the showrunners hit upon the idea that the Doctor could physically transform and renew his body, which allowed them to continue the show after Hartnell's departure in 1966.