While I don't have a definitive answer, I would assume it's because churches are primarily (financially) supported by their congregations. The UK, like most other countries in the world have a shrinking rural community compared to the urban centres. If the money isn't in the community, the church will be left to ruins.
Especially if we are talking about post WWII, I would assume that there were more important things for the money to go towards, like housing, farming and industrial infrastructure, and roads/railway lines etc.
From my limited understanding, once a building has been left in ruins for more than a year or two, it's a huge job to fix it up again and would not be possible without a benefactor, or governmental support. Probably easier just to build a new (probably less grand) church, and move on.

I agree that they are marvelous buildings, but I'd assume it's mostly financial, rather than a sectarian reason (which would probably be outside of the rules of this forum to discuss).

Just a thought, hopefully someone can give a more definitive answer.

Cheers,

Cameron