I think this actually is a historic issue from the inception of BBC2. Back in the black and white days in the 1960’s tv in the UK was broadcast on VHF bands I and III using 405 lines. These transmitters had a very large transmission area and so only a handful were required to broadcast to the whole UK. This was true for both BBC Television and Independent Television, each with one channel.
It became apparent that the future of broadcasting in the UK would necessitate a transition to narrower UHF wavelengths, which would give a better quality picture, and permit more channels to be broadcast. The BBC and ITV channels were already making plans to start dual broadcasting on the old VHF and the new UHF. At this time BBC2 was born, and it was decided to launch on UHF wavelengths only (625 lines) since these were the future of UK broadcasting, indeed lasting us up until the inception of digital television in the 2000’s.
For a time dual frequency TVs were available. These were expensive and unreliable. Older readers may recall the newest models in the 1970’s had ‘push button’ tuning instead of a rotary tuning dial, and these were labelled BBC1, BBC2, ITA1 (or ITV1) and ITA2 (or ITV2). The ITV2 commercial service never materialised, and was eventually launched by the government as Channel 4 in 1982.
UHF transmission required many more transmitters and relays, since it has a much smaller transmission area. This made matching the coverage area for the old VHF transmitters a lengthy and costly process, which took almost 20 years to complete. The final switchover from VHF (405) to UHF (625) happened in 1984 and early in 1985 the old 405 system was switched off. Those frequencies today are used for DAB radio transmissions.
The upshot of all this was that until 1984, not all of the UK could receive UHF transmissions, and therefore BBC2. So naturally if you had a more niche programme, it would perhaps be relegated to ‘2’ where not everyone would receive it, but many might not be bothered. Conversely programmes of national importance, and mainstream programmes would be broadcast on ‘1’ so that everyone could watch, whether they were watching on UHF (625) or VHF (405)
This is why programmes which became more popular than envisaged would transition from ‘2’ to ‘1’ and conversely programmes where audiences were perhaps declining would be parked on ‘2’ before eventually being axed entirely.
Which all serves to explain why BBC1 was and is the flagship channel
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