Bell's book says that he and Clarke didn't initially think the play characters were worth bringing over, but as Adanor said, they liked the chemistry they had when Howard and Pearl were recast for the second year run of the play. By that time, they had also lost John Comer, and I do think Clarke was looking to change things up. For several series it had just been the trio plus Nora, Wally, Sid, and Ivy as the regulars. On top of that, Clarke kept playing with the right way to add a humorous take on adultery, going all the way back to the pilot. Not to mention a few episodes where it was implied the Sid had cheated on, or tried to anyway, Ivy.
I've posted this before, but things I've read over the years suggest pretty strongly that Clarke got bored with the basic premise of the show after ten years or so and I think adding new characters was his way of keeping thing interesting. I do not believe the show would have lasted as long as it did if he had not been allowed to do that. There is only so much you can do with the trio formula he established at the outset. Very few comedies last ten years, let alone 30, because eventually the core premise gets worn out.