John Quiggin asks, “Why hasn’t Labour introduced preferential (single transferable) voting in Britain?” It’s actually a fairly good question, although I think Quiggin answers it later in his post:

Sooner or later, there will be a hung Parliament, and the price of LDP support will be full-scale proportional representation. If Labour introduced preferential voting without being forced to, it would not only cement LDP support but would greatly weaken the case for PR.

Labour, however, doesn’t need to make a deal yet—and, judging from the past 100 years of British electoral history, a hung parliament where Labour needs the LDP either to form a coalition or to sustain a minority government isn’t likely to come about anytime soon. So why help the LibDems today if you can put off an accomodation until later, perhaps much later?