I think it's because these projects are usually 80-100% done, and the riskiest part that's left is the print run. Potential customers are given sample PDFs so they can see if they will like the final product, and there is significantly less risk compared to a video game or something with a more complex mechanical design.

It effectively acts as a pre-order system for many projects, letting them know how many to print. The more you print at once the cheaper things are - print on demand is very expensive even for books. Many of these projects do one print run to fulfill the kickstarter then toss the pdf over on drivethrurpg.

The project I linked is unique as far as projects I tend to back goes. It's an RPG system where they haven't even figured out advancement yet. There's going to be more inherit risk due to that, but it's being ran by a big name in the community with experience in delivering these projects.

Contrast this with Shadowdark: it had design, art, layout, etc. ready to go. All they needed to do was the print run. https://www.kickstarter.com/projects/shadowdarkrpg/shadowdar...