That's because there hasn't really been any hard evidence. From the article itself:

It has four different levels of classification - carcinogenic, probably carcinogenic, possibly carcinogenic and not classifiable. The levels are based on the strength of the evidence, rather than how dangerous a substance is.

"Probably Possibly carcinogenic" is thus the least supported one one can make a ruling with.

Then it all depends on the studies themselves too. Like one study on sunscreens found that oxybenzone caused endocrine disruptions in mice; when force-fed unrealistic amounts of it. Like what does that even tell us? Don't compulsively eat sunscreen, you could get sick?

The chemical was prohibited nonetheless, because generally a "better safe than sorry" approach is taken. These corporations don't want to face massive class-action lawsuits, so you can expect aspartame to be phased out.