It's not meant to be an exclusive list, he was just the most famous billionaire in the UK (where I grew up) while I was growing up.If I look at e.g. this list: https://en.wikipedia.org/wiki/The_World's_Billionaires_2015 (chosen because it's the most recent one on Wikipedia that's not summarised to a top 10), most of those people I've never heard of and know nothing about. I recognise perhaps a fifth of the names, know little about even most of the names I do recognise, and and that includes the ones (Musk, Bezos, Murdoch, Zuckerberg, the Koch brothers, and Ellison) who are infamously colossal dicks.
Richard Branson, in the UK, is mostly known for being a serial entrepreneur and using the word "Virgin" in all brands even where this is silly (Virgin Maternity Wards, Virgin Brides).