More or less, the offer from both senior parties to the LibDems depends from the same flawed premise. Both parties are prepared to put AV to the country; neither are prepared to do the same with STV. And I believe the reason why is pretty obvious.
It’s about safe seats. AV is the only one of the alternative systems which preserves them intact. Safe seats are graphably the reason for the expenses scandal. Safe seats allow parachuting of candidates, placing too much power in the hands of central committees over local candidates and parties. Safe seats are wholly counter-democratic. And the LibDems have almost none of them, but the other parties have quite a few each.
For voting reform to matter, it must remove safe seats. Only one system does. Neither large party can countenance that easily. 62% of those polled recently support PR. Therefore neither large party can offer a referendum on anything other than AV, which isn’t PR.