WotC doesn't have forums anymore. That's now how most people engage with brands online anymore. I'm not going to fault WotC for eschewing forums than I will fault Paizo for eschewing Usenet….
That was my thought. 75% of good mechanic design is coming up with the cool idea, especially in 5e. The rest is easy to teach.
This also assumes the "better candidates" applied. It does require moving to Seattle, and many other designers have work or a day job they like.
Plus, we haven't seen her resume. We have NO IDEA of her full qualifications, just the stuff she listed on her artists' website.
Trust also comes into it. Do we trust WotC to hire someone who can do the job?
So far they've done good with 5e. I cannot imagine Perkins, Mearls, Crawford, Lindsey, or Stewart signing off on someone they don't believe is able to put in the work or will harm the game. To them, D&D is as much a passion as a job, and I don't think they'd tap someone who would make bad products while also making their job harder.