Right-wing media smear campaign? Split vote on the left? Lack of confidence in Clegg's leadership/not wanting to afford him an outright 'kingmaker' position? Local council irregularities? Gordon Brown picking up steam in the last few days? The Conservatives switching their campaign strategy to effectively lock down the surging support behind Nick? Immigration perhaps being a pivotal issue on which the Lib Dems were the wrong side of the general will of the country? A system which badly disadvantaged him from the off?
All of the above and more?