With leaders of both parties being pro brexit (At this point) you would think the Liberal Dems would poll much better
Because The Very Clever and Serious People running the party (Clegg and the Orange Bookers) decided to nuke the party's previous voter coalition from orbit in the belief that they could turn themselves into FDP 2.0, which was - and is - a recipe for oblivion under FPTP. It's symbolic that the two party leaders who painstakingly resurrected the Lib Dems have both passed away in the last couple of years.
Huh the Lib Dems never had a coherent voter coalition in the first place, they were the 'not Tory, not Labour, all things to all people' party so unsurprisingly when they actually had to govern they completely imploded. Look at some of the constituencies they held e.g. Burnley, Sutton and Cheam, Hornsey and Wood Green, Torbay. There is virtually no ideological overlap between these places whatsoever. If they had gone into coalition with Labour they would have collapsed just as badly, if not worse (seen as most of their constituencies were in centre-right areas) there is no way the rural voters in the southwest or the bourgeoisie of Southwest London would have put up with the LDs supporting a left wing government.