There are plenty of Black and Asian cylists on the L2B for instance, I first cycled it with a Sri-Lankan guy in 1982 azndf there were some blinging black club riders back then, so they've been around for some time just not necessarily in big numbers. Why there aren't more I'm not sure. Some years ago on C+ there was a similar thread and IIRC it was agreed that in all "outbound" style pursuits, hikong, canoeing, camping etc there were fairly few people form the UK's ethnic minorities, but I'm certain that is changing albeit slowly. However in athletics there is a very high percentage.
Whether it's a class, money, cultural issue I do think it's changing.