No one's saying it wasn't successful. The first movie made about $875 million worldwide, which is great for a new IP. The "problem" is that the second film made about $429 million worldwide, less than half of what its predecessor did. It still seems like - and is - a hell of a lot of money, but it's problematic (for lack of a better word) because it shows declining interest on behalf of the audience. Couple that with the film's lukewarm critical response. The sequel was greenlit not long after the first film's release and success, whereas it's been months since the sequel released and there's not a word about a third film, which just suggests that those involved are likely mulling over what to do going forward (if anything) when there's a risk another sequel could suffer another 50% drop.
And I'm not commenting on whether any of that has to do with whether or not a ride/land could be great. As others have pointed out, it's not like the Cars franchise blew up the box office compared to other Pixar movies, yet the land is great. I've never seen either of the Pets movies, but I'm holding out hope the ride is good... It'd give me a reason to go to that area of the park more since I always skip Minion Mayhem every time I visit.