There are various factors that lead to this. One is the "first past the post" system - where the person who gathers the largest number of votes in the constituency is elected the MP, and the party with the largest number of MP's is the governing party. This is like throwing away the votes of everybody in a constituency who didn't vote for the elected candidate - their opinions simply become irrelevant from then on.
The nature of this system also discourages turn-out, and distorts the election campaign. In most constituencies, there is no chance of the majority of the incumbent being overturned. So why bother voting, if it's not going to count for anything? Also, more so than in any previous campaign, the focus of the two main political parties has been on marginal constituencies - the 10-20% of seats where the majority was smallest - because if a party were able to get more control of these seats, then they would most likely be able to swing what happens in parliament. So the concerns of 80-90% of the electorate become irrelevant to the main political parties.
The net result was that many people felt that there was little to choose between the parties in terms of their opinions being represented, and many people, lacking a candidate who they felt represented them, ended up voting for somebody they thought was the "least worst".
Finally, because in excess of 50% of votes are irrelevant after polling day, there is a vast variation in the number of votes cast per MP returned between the political parties. On this blogsite, somebody has done the calculations:
lab 25968 votes per mp
con 50347 votes per mp
ld 92554 votes per mp
But once again, how is electoral reform likely to happen? The current system ensures a disproportionate level of power to both the dominant political parties. Are they really likely to have any desire to change the system?
See the Make My Vote Count website here