It's political. There's no parliamentary support for a general tax, but no public support for getting rid of the BBC. The Tories in particular hate the Beeb, but the public love it (contrary to the rather negative comments here).
So the tax lives on in this archaic form instead and every now and then when the Tories are in power they try and defund or threaten the BBC somehow. Usually within a year of getting elected, and then there's a backlash and they have to back down.
The latest wheeze was to force the BBC to get pensioners to pay, and they managed to pull it off by getting the public to blame the BBC instead of the government.
So the tax lives on in this archaic form instead and every now and then when the Tories are in power they try and defund or threaten the BBC somehow. Usually within a year of getting elected, and then there's a backlash and they have to back down.
The latest wheeze was to force the BBC to get pensioners to pay, and they managed to pull it off by getting the public to blame the BBC instead of the government.