But you're buying a niche product aimed at a tiny demographic. The regular XPS 13 is extremely popular and I'm sure they sell in the hundreds of thousand (given how many businesses/governments buy, not to mention private consumer sales). So as a private consumer buyer you're benefiting from the economies of scale when you buy a Win10 XPS 13.
Unfortunately someone has to pay for the Ubuntu install/testing/drivers and Dell ProSupport who has support staff who can help with Ubuntu (unlike regular support who only do Windows).
PS - ProSupport is an enterprise support product. The regular XPS 13 comes with "Premium Support" which is a consumer product, it is essentially normal support but they'll also help you with networking equipment, and popular third party [Windows] software. With Premium Support you have to buy SupportAssist to get any on-site support, ProSupport has it out of the box (as well as next day turnaround on parts).