> Why?

Initially because it seemed like a good idea at the time. Then the audience of the pilot episode liked it, so the Beeb overrode the loudly stated "Never Ever Again" from the cast* and the host**. I can't remember whether the pilot included "the game called Mornington Crescent".

* Basically the script-writing team of I'm Sorry I'll Read That Again (qv).

** The incomparable Humphrey Littelton, purveyor of breath-taking single entendres.