Hi Suz, and I apologize for some of the rude responses above. When I was first getting into chipping I also didn't know much about Paulsons either or what made them so desirable and pricey. To sum it up simply, Paulsons are no longer readily available like they were in the past as GPI (paulsons present parent company) has pulled out of offering chips to the home market. Aside from that, many chips are limited in production, have different colors and edgespots that appeal to different people, have historical or personal meaning to people, etc. Rarity is something that usually drives up the price of a chip as well.
Why are they so great? They are used in many actual casinos making them "real" casino chips. THC (top hat and cane) have indistinguishable feel/sound and are generally considered the top end chip you can get in the world. I'm sure this doesn't cover everything but I hope it helps