The only reason we got the referendum at all is because the Conservatives didn't win an outright majority of seats, and so weren't able to form a government after the last election. They had to form a coalition with the Liberal Democrats, and the LD's terms for lending their support to a party that they have very little in common with was they they insisted on the AV referendum. The Conservatives grudgingly accepted this, but would never have agreed to a referendum on full PR.
Basically, the entire thing is, sadly, driven by self-interest. The Tories don't want AV because they will probably lose some seats under it; but they DEFINITELY don't want PR, because they (and Labour) definitely would lose seats to the Lib Dems.
... and, lest anyone think that losing seats to the LibDems just means it's the LibDems' self-interest driving their wish to change, look at the comparison of the vote and seat percentages. At the last election, for every LibDem MP elected there were 5 Conservative MPs elected, but for every 2 LibDem voters there were only 3 Conservative voters. Is that really the parliament we asked for?
We need AV not because it's the best forever but because it's our only chance of reform for a generation. I'm voting yes.
Basically, the entire thing is, sadly, driven by self-interest. The Tories don't want AV because they will probably lose some seats under it; but they DEFINITELY don't want PR, because they (and Labour) definitely would lose seats to the Lib Dems.