It's just not going to sway nearly enough people to win a majority, and the Tories know that.
That decision potentially alienates about 1/3 of their previous voter base who voted to Leave, as well as those who did want to Remain but either don't believe in overriding a popular referendum on democratic principles or don't feel strongly enough on the issue to disrupt things further.
Given that we're talking about the Lib Dems here, all of those could be significant groups. It's quite a gamble to bet your whole political strategy on attracting enough voters from other parties because they feel so strongly on Europe, and according to recent polling, it looks like it's a gamble that is going to fail unspectacularly.
FWIW, I know from immediate personal contacts that it does, and not in entirely trivial numbers. I'm in one of the most pro-Remain areas of the country, but also a LD/Lab marginal, so it will be interesting to see what happens as a result. I suspect it won't make much difference here, but if the same effect is evident in more moderate parts of the country, it could actually hurt them.
It looks like there's been another poll today that is consistent with what I've seen reported previously: about 1/3 of Lib Dem voters at the last election voted Leave in the referendum, and currently Lib Dem loyalty is lowest among the major parties, with a similar proportion of Lib Dem voters at the last election expecting to vote for someone else at the next one.
Without such a gamble the LibDems will remain in the also-ran group with UKIP and the Greens. You push any issue that gives you a wedge and use it to crack a few ridings and if necessary you write off the 1/3 in the hope of picking up 2/5 of the two larger parties.