Why did the Slam Dunk anime stop without animating the Interhigh tournament?
As per here, (not sure how credible the source is but it makes sense). So echoing back your thoughts..


  This series, as some anime are known to do, was cancelled after the
  initial 100 episodes. Most of the time this happens because the
  ratings are not that good, or the manga has not been finished at the
  time. I believe the reason for Slam Dunk not to continue had to do
  with money issues with its developers ...


and the answer also makes mention of the manga


  Concerning the manga storyline it goes along the lines of the anime.
  And yes, the manga continues into the Inter High tournament, right
  until it is finished. It has a open ended finale, but in my opinion
  its one of the best anime endings I have seen. If you enjoyed the
  anime, reading the manga is a must.


Following up this answer, it seems to be a thing. Unfortunately I can't seem to find any ratings, viewership numbers or official source to back this, but for the majority it coincides with what you presumed in your question.