https://www.bbc.co.uk/news/articles/c4nglegege1o
Reform's seats came from the Tories, unsurprisingly, and like you say Reform won more of the popular vote than the Lib Dems (4,117,221 vs. 3,519,143; not a wide margin) but Reform also campaigned in many fewer constituencies where they didn't have to compete directly with the three largest parties (not to mention Lord Buckethead and the Monster Raving Loony party, their nemeses). So maybe they have lots of supporters in certain areas, but only in those certain areas.
Reform is not a serious political force in the UK. They only renamed themselves from The Brexit Party, but they remain a single-issue party that appeals to a tiny minority of voters. The majority of the electorate are much more concerned with real issues like the economy, the NHS, education, law and order, and the environment. Brexit wasn't even a particularly big issue in the last elections. Even the Lib Dems, who had campaigned for a second referendum in 2019, laid it to rest this time and focused on more recent issues like sewage spills in rivers etc.
Might I also hog the mic a little while longer to say that I, personally, am mostly socially conservative, and am absolutely appalled both at the Tories and Reform, who are nothing but right-wing populists and demagogues that do not care a jot about all the things that socially conservative voters care for: jobs, order, stability, lawfulness, the economy, family, etc. And let's not forget that it was Margaret Thatcher's Tories that got the UK into the EU, and did so because it was beneficial to the economy, trade, and the stability of international politics. Exciting the EU was exactly antithetical to conservative ideals: it was a radical act of self-mutilation.
Labour are now the conservative party, the party of business and fiscal responsibility (and sitting on your hands while you kick the can down the road) and that's why they took all the Tories' votes: because the socially conservative constituency got fed up with the Tories' antics and, the Brexit fever having passed, wanted to go back to order and stability.
In the 2024 UK general election, they got 14.3% of the vote. I don't think that's a "tiny minority". And if that's a "tiny minority", then the 12.2% of the vote Lib Dems got is an even tinier minority.
The problem that Reform has, is that 14.3% is spread too thin geographically. Reform got 5 MPs from 14.3%, Lib Dems got 72 MPs from 12.2% – because Lib Dem support is more concentrated in particular constituencies, mostly in southern England.
This is a side-effect of first-past-the-post. If the 2011 Alternative Vote referendum had succeeded, then Reform would have likely ended up with more seats in 2024; although even with alternate vote, the seats-per-vote advantage that Lib Dems have over Reform due to their greater geographic concentration would have still existed, albeit somewhat attenuated. A more fully proportional system, such as those used by the devolved legislatures of Scotland, Wales and Northern Ireland, would have attenuated that advantage even further–although those systems are still region-based, so even they may not completely eliminate it.