Because they have enough data to form a hypothesis that requires more research to confirm. Even if you leave the "may" out it'd be very vague because we don't know what exactly causes the cancer. Pesticides on vegetables is just their best guess and even if that's correct, which ones exactly? It might also be something else entirely that happens to correlate with a veggie heavy nutrition but has little to do with the vegetables themselves.

Uncertainty isn't a bad thing.