For the same reason that people who despise right wing ideas will call anyone who doesn't agree with them a Tory or the classic in the aftermath of the election, calling the electorate stupid for what they did
It doesn't make a jot of difference but if there is something people don't like or...
People who say "we need to go to the left to win the election" see 1997, 2001 and 2005 as defeats and 1983 and 1987 as victories
When was the last time this country elected a left wing leader? Even Miliband was too far for the electorate and he looks like Thatcher compared to Corbyn