Because it has no bearing on the science of climate change, and it's very emotive -- a lot of people get the red mist if you mention the name Al Gore. (Note that I'm not American so this is just my impression from afar, but in a UK context you could substitute Thatcher or Blair as politicians that are so polarising that reasoned debate becomes very difficult and inevitably gets bogged down in very angry arguments about coal miners or Iraq).

It might be relevant if we were talking about carbon credits or some specific policy that Al Gore is involved in, but the rest of the time it's just going to take the discussion in a bad direction.

It may seem like I'm saying 'ignore the problem and it will go away' (the problem being Al Gore's hypocrisy). But there will always be people involved with or supportive of a cause that one can find negative things about. Expecting purity from everybody who just happens to agree with you, especially if they are politicians, means you will be waiting forever before you get anything done.