There's a very good reason, it would cost a ridiculous amount of money.
PR or any other system would require a full root and branch change to our electoral and parliamentary system or it would be pointless. It would also require a long consultation process, public education campaign and probably multiple referendums, just to decide which type.
There's also no guarantee it would change voting habits or produce a functional government that isn't brought down every 5 minutes.
It cost £75 million, just to hold the AV referendum.
The cost to the taxpayer of Nick Clegg's bid to ditch first past the post - which was roundly rejected - is revealed.
news.sky.com