From everything I've read its a result of the culture at Google, where promotions are given to those who come up with something new, not to those who take something existing and make it great.
I've also used it for video calls. We had connection issues and swapped over to Houseparty (!) to get group chat working fast. That was stable as a rock. Excellent. And we played Uno during the meeting. Bonus.
But Duo seems ok now. For video calls we now use it and are happy enough. So, take that for what its worth. We'll continue using it until it is mysteriously cancelled next year.