I think the reason is timing as far as DT and Uconn coaching. We don't know how long Geno will continue to have the fire, but you figure DT will be playing 5+ more years. There is no way Uconn would hire someone with no coaching experience to lead the program - so you need to expect Geno to hang in for something like 10 years before DT would have the coaching experience necessary. And I don't think DT has ever actually said when her playing days are over she is going to coach.
Geno may coach for 10 more years, but I think that is probably the outside limit, and I wouldn't be surprised if he doesn't last the full 10.