Who's competing with what, exactly?
Firstly, dominance of a couple of brands is hardly surprising when only two of them have actually existed for more than a couple of years. Sky has been around for over 25 years and Virgin have been around for nearly a decade (in both cases as mergers of older companies). In comparison, BT and TalkTalk have only existed as TV providers for a couple of years. You don't get to start up a brand new company and immediately start complaining that the well established competition has more customers.
Secondly, the market share claims appear to be based on entirely arbitrary and meaningless distinctions. Sky operates broadcast TV that is only really similar to regular free broadcast TV. Virgin, BT and TalkTalk all operate internet TV which functions in a completely different manner. Given that TalkTalk doesn't actually have its own hardware and operates over BT's network, there doesn't seem to be any good reason for having these four listed as the only pay-TV providers in the UK (not counting a couple of small regional ones), since there's no meaningful difference between them and Netflix, Amazon, and so on. There's no difference between having a BT or TalkTalk internet+TV contract, or BT internet + Netflix contracts separately.
So the 64% claim is really nonsense. Either Sky have a 100% share because they're the only ones actually operating broadcast pay-TV, or they have a much smaller share because they're competing with a bunch of other services that just aren't being counted for no apparent reason.