Needless to say, Jim Murphy was/is the epitome of New Labour, along with attempts of his own to appeal to the McTernan-invented "Glasgow man" by appearing in footie strip at every available opportunity, as if that made him a man of the people.
I'd say a long history of Scottish Labour turning its back on Old Labour is closer to (at least one of) the true reasons.
I think the Lib Dems had a history of benefiting from traditional support in more rural areas, along with benefiting from tactical voting they didn't recognize as such (in my own constituency, up till this election, for many years it's been a clear fight between the Tories and Lib Dems), and the ability to try to be all things to all people depending on the constituency (all parties do that to a certain extent, but I'd argue it's long been more the case for the Lib Dems).
The coalition with the Tories scuppered all that, especially as they didn't seem averse to doing it all over again if the opportunity arose.