It was released in the same time period as the first film and didn’t really have much in the way of family-friendly competition, unless, of course, families were waiting and prioritizing How to Train Your Dragon: The Hidden World at the end of the month, which did perform.
From my layman’s perspective, I don’t think it was a failure of marketing because the movie looked fun just like the first.
It’s honestly tough to say, and I’m sure Kevin Tsujihara has experts who get paid a lot to figure out the answer to that question.