Of course, this is the ONLY Nicholson Joker product with his licensed likeness, they know they have collectors by the balls. Even Batman, there have been products with Keaton's likeness but not a lot.
I talked to a friend in WB PR about the Nicholson deal. While it's never been publicly confirmed, Jack basically struck gold because Burton & WB were so desperate to have him. He agreed to cut his customary front-end salary at the time in half for a percentage of the movie's gross. Also, he gets 5% royalties of all licensed products from the film... and 10% of anything with his likeness on it.
That's your difference right there and almost surely why there haven't been many Nicholson Joker products (or even a lot of '89 BATMAN products in general) over the years.
He also did get a small percentage of box office & merchandise from all Batman films made between 1989-1999. It's been said that he made over $60 million from the first film alone, and possibly as much as $100 million overall.