Why is it so hard to find a good used Suburban 2500

Probably 5% or less of the Suburbans from 2007 to 2014 were 2500, so that adds to the price.

As APT suggested, a 3500 Express makes a great towing vehicle. I can tell you that I get 60% better fuel economy not towing between my Express and my Yukon XL that had the 8.1. Also, you can fit more people in the Express.