IMO this was one of the worst political catastrophes in 21st Century UK politics. Young, educated people formed a core part of their voter base at the time. This demographic felt betrayed by the Lib Dems propping up a Conservative government and a tripling of university tuition fees, increase in VAT, and introduction of austerity. Clegg also failed to lead an effective campaign in the "Alternative Vote" referendum, resulting in an overwhelming defeat and decades-long setback for voting reform. (N.B. there are interesting parallels between this and the Brexit referendum. In both cases, the opposition was allowed to spin it as personality politics.)
Only now has the Lib Dem vote share recovered, largely due to an implosion in the Conservative party and a relatively regressive Labour electoral campaign.
He gave the largest party a majority in exchange for a referendum on first-past-the-post.