He's just an easy target, and it's a classic debate tactic. When you don't have any substance behind your arguments, you have to resort to another strategy. Ad hominem is an easy one - attack the opponent instead of his argument. There are other such strategies which global warming deniers often use, the main one being stawman. Construct an argument which your opponent isn't making but which is easy to disprove, pretend he's making it, then disprove it.
Al Gore is an easy target because he's a politician, so they can pretend global warming is political. He's not a scientist, so they can pounce on him when he makes a minor error when discussing climate science. He travels a lot and lives in a large home, so they can criticize him and pretend that all global warming 'believers' are just hypocrites. They can point out how much money was made from his movie (even though his proceeds were all donated to a non-profit) and claim he's just in it for the money.
These are all arguments to distract from the fact that global warming deniers don't have any valid scientific arguments. So instead they engage in pathetic distraction tactics like ad hominem attacks on Al Gore.