Why does Party Leadership affect the Electoral Outcome? [Tony Blair and Jeremy Corbyn]

-Blair had Huge Approval Ratings which led Labour to get Huge Wins in 1997 and 2001. But this tanked when Blair went to War with Iraq, showcased in their Majority tanking after 2005 GE. -43% of Labour Voters had voted based on 'Leadership' which shows their Dislike on Corbyn. He may be Blamed to how the 'Red Wall' [Labour Safe Seats] wee Shattered by the Conservatives