Frank Azor cites low interest in gaming subnotebooks like the Alienware 13 as a big reason for skipping additional GPU options for the XPS 13 family. A few competing 13-inch Ultrabooks like the HP Envy 13, Huawei MateBook X Pro, and Razer Blade Stealth all offer optional GeForce MX150 graphics for superior graphics performance over the XPS 13.
https://www.notebookcheck.net/Dell-has-no-current-plans-for-another-XPS-13-with-Intel-Iris-Pro-graphics.386476.0.html