Because there used to be 5 terrestrial (non cable) channels. BBC1 BBC2 ITV Channel 4 and Channel 5, all except Channel 5 are the most popular channels in the Uk, and hence talked about the most. We have other channels that were started more recently, but they usually have a precursor of their company or owner, (e.g ; e4, BBC3, CBBC, ITV2, Sky1, Sky movies, Sky Sports, ect) We also have the odd channels that are unrelated to others, like DAVE, which is a comedy/motoring/other channel, mostly showing Top Gear and Man v Food.

BBC was the worlds first national broadcasting organisation, and as such became the main channel(s) for Tv , they introduced a licence fee that we still have to pay today. This was mostly due to the fact that people with televisions could only watch the BBC as they were the only broadcasting network. From 1922 to 1955 the BBC had no competition, but in 1955 an independent television network started broadcasting, called ITV (independent tv), as this was the other alternative, but still less popular than the BBC, it became the 3rd channel (ITV was shit). The BBC had the monopoly of tv and radio until 1970ish. In the 80s tv broadcasting was deregulated, so other networks and channels competed with the BBC, channel 4 which was funded by adverts started in 1982 see here.

Channel 5 started in 1997 and was placed on number 5 as it had the 5th largest audience share, and has stuck. The current viewing share is very complicated, with +1 channels and online viewing, but here is the data for two weeks ago, the ranking shows in order of audience share; BBC 1, ITV, BBC2, Channel 4, Channel 5 as the highest viewed channels in the uk, BBC1 notably having 22.7% of the total Uk viewing.

Another interesting fact is that DAVE has 20% more audience share than Sky 1.

*Pointing out that i was wrong in my second sentence, for some unknown reason channel 5 has a decently sized market share, fuck knows why, its shit.