While it had a very good US/Canada opening weekend of $68,532,960, for its second weekend it made a total of $28.5 million. That's a significant drop off and it continued to bleed ~50% of its box office every weekend it was in release.
It also cost $100 million (estimated) to produce. Add a...
A sequel was planned *if* the movie performed well enough. It did not.
When asked if he'd be interested in working on sequel, Burton replied, "I'd rather jump out a window."
https://en.wikipedia.org/wiki/Planet_of_the_Apes_(2001_film)#Cancelled_sequel