Looking at the actual voting figures it is apparent that it wasn't a matter of lots of people who voted Labout being reluctant to vote for Ken. His acual vote was pretty well in line with the Labour vote generally, a vote which resulted in a landside for Labour in the London Assembly.

The trouble was that Boris got a much higher vote than the Tories generally - probably because a lot of non-Tories see him as a laugh. The same way that when elected mayors were introduced in 2002 the voters in Hartlepool elected a man dressed as a monkey. (And in this picture he actually looks quite a lot like Boris, but with a darker mane.)