Because it depends on geography as voting in the UK is based on who you vote for your local MP. If UKIP's four million votes were entirely within 20 or 30 cons

Because it depends on geography as voting in the UK is based on who you vote for your local MP. If UKIP's four million votes were entirely within 20 or 30 constituencies, they'd have won them all. But because their votes are spread out across the entire country, other parties pick up those seats instead. The parties that do best are either those that get a ton of votes everywhere, or those that dig in and solely focus on specific locations where they can get a majority (the Lib Dems used to be good at this).