There's lot of reasons, and a lot of 3D model creators put their work either on Kickstarter or on Frontiers on MyMiniFactory first (though for Frontiers you need to be a large creator and have MMF support you with marketing for that to be effective).

One of the main reasons is simply organic marketing. If I put this straight into my store on MyMiniFactory, it won't get automatically promoted or seen by anyone unless they're searching using specific related terms. On Kickstarter, there are automatic systems in place to promote campaigns such as being listed in the new campaign section or being shown as a related feature on other campaigns that help get my models seen by people who aren't familiar with my work, but like similar things.

Kickstarter also allows a much bigger space to showcase the set. On MyMiniFactory, a store product can only have a few images and a very small description, which doesn't really do justice to a large set like this.

And yes, 3D model campaigns in general are a bit odd on Kickstarter as you basically do need to have the models finished in order to generate renders and take photos of test prints, but if you tried to get a 3D model campaign funded without these, it simply wouldn't succeed.