I think she'll direct another episode this season, so maybe then?
While I didn't know Thompson's directing resume, this makes a lot of sense. Given her husband's background, and the all-too-common decreasing opportunities for women past a certain age, going behind the camera is a viable option (if still a mostly-boys club).
I don't have a sense for the behind the scenes operation of this era of Trek television like I do for the earlier spinoffs. It was certainly nothing out of the ordinary back then to see a lot of the same names directing so-called genre shows. But I don't know that those directors were sought out from a story perspective more than a logistical perspective. Having a stable of regular, rotating directors just makes sense, regardless of the nature of the show.
Even in the current golden age of television, TV auteurs don't direct all that much. Those that have (Sorkin, Abrams, Whedon) usually do more film directing.