I don't know much about Kelley, but if the goal is to create a "C replacement", aggressively keeping the language surface area small seems like a reasonable way to go.
https://youtu.be/Gv2I7qTux7g?t=120
All these programming language features require a lot of machinery, complicating the binary interfaces of the resulting software and preventing code reuse.
Also for instance Go has been able to stay relevant while staying very small. Complex languages can help with complex projects if there is a good program structure, but one advantage of simple languages is there is only so much of a mess which can be made if developers go a bit off the reservation.