Arguably, because :-
- the BBC is a public service and we all (if we watch TV) pay for it. We can then choose what, and how much, to watch.
- because given current analog technology and current TV sets, it would be hard or impossible to restrict, without forcing everyone (including those on fixed incomes, like pensioners) to buy extra hardware
- because, as a civic service, it's like many things we all pay for, whether we use them or not
- and perhaps because the costs of and administrative effort in taking it subscription-based, in the short term, would be prohibitive.