Why did Democrats lose the last two elections?

The answer to that is easy - because they nominated losers.  

But to be more specific, they lost by nominating someone from the extreme left.   Why don't they nominate someone more centrist?  Gore was a huge liberal.  Kerry was left of him.  And now Obama is left of Kerry.    Do they think that going farther left is a good strategy?  There are good democrats that are not among the Kennedy, Biden, Hillary crowd.  Why can't they fix their system so they can nominate one of them?