The problem is there is a difference in bias. Ostensibly a journalist is interested in following what leads to facts while suppressing any personal bias. Activists seek facts which support their biases (as well intended as they may be).

You're forgetting that pursuit of facts and truth is not necessarily orthogonal to "activism". Can I be a climate activist journalist while still uncovering facts about people trying to suppress knowledge of global warming and poison the well? I don't see why not.

One hopes a journalist isn’t selective about the facts they present. For example only include facts which might contribute to incriminate someone but omit facts which could exonerate someone.

The concept of activism itself is neutral. And useful, and important. In principle.

The difference between an activist deciding arbitrarily on a position through bias vs. coming to a position legitimately and promoting it is largely in your perception and based on your agreement or disagreement with their position.