Because the British First-Past-The-Post electoral system provides no safety valve for dissatisfied or disaffected voters. As Andrew Gamble points out in a seminal article on “The Realignment of British Politics in the Wake of Brexit”:
Other third parties have had bursts of success, but have not been able to break the stranglehold of the two main parties. The most recent example is UKIP. Despite its success in winning more seats in the European Parliament than any other party, and winning four million votes in the 2015 general election, it only won one Westminster seat, and that was a seat held by a Conservative defector. If seats had been allocated proportionally in 2015, UKIP could have expected to win more than eighty. The main impact of third parties has been to reshape the policies, leadership and electoral strategies of the two main parties, rather than to replace them. Could Brexit change this?
Four million votes — and one MP.