You can watch the first minutes of this interview with Travis and Matt.

The short answer seems to be: The decision to use D&D was made months earlier and they were too committed to that already.

Based on interviews that I’ve seen, they didn’t go with daggerheart because campaign 4 has been in the works for over a year, prior to the daggerheart release. So they had to develop it around an existing system. Also, their campaigns are the main part of their brand. I would assume they wouldn’t want to go all in on using a system they weren’t sure was going to even be popular.