The issue is - comedy is the province (to a large extent) of the young. Younger people are more likely to lean to the left, so the comedy market reflects this.
Comedians that are favoured by older people (Sarah Millican, Peter Kaye and Michael McIntyre) are noticeably politically neutral.
If you tune in to Mock The Week, and they rely on the most popular British comedians, it will be populated by those who lean to the left.
I don't think the audience really wants to watch comedians that their grandparents favour.