I've just begun querying on a book-length satire project myself - my impression is that it's not so much that satire is hard to sell, but "satire" isn't really a well-defined genre in the book market.
There's a humor section in the bookstores, but it's not a genre where ALL the books on the shelf might appeal to EVERYONE browsing it. "Humor" means gentle anecdotes to some people, barbed satire to others. The topic's currency, associated public interest level, and the nature/tone of your take on it have a lot to do with its marketability. The marketing aspect actually strikes me as closer to non-fiction than anything - identify the audience who will "get" and appreciate the joke, and focus on them.