Everything I've read suggests Champions was cancelled due to sales. Most of the series' existence was spent as a bimonthly book (every other month), which is what Marvel did with low sellers back then. There was an attempt to generate interest just before the end, by guest starring in other titles, and when that failed, the plug was pulled. The ending was sort of abrupt, and the team didn't disband until after the series ended.
The series had many problems that caused the low sales. There never was a good reason for the team to be together. There was instability in the creative team (like a dozen different combos between the 17 issues). The cast probably played a role, too. And no really good stable villains.
It's funny, in my youth I loved the Champions. Just the idea of such a diverse, downright bizarre mix of a long time Marvel heroes coming together tickled my adolescent funny bone. But looking back, they were poorly written and as you said, Guardian, never given much to a motivation. How many times can a team fight Swarm?