It's just a difference in use. With the starters everybody had one, and so they had to be balanced amongst themselves (well, not counting how easy it was for water pokemon to have ice beam).
All the other trios were more individual in nature. You weren't expected to run around with a team of legendaries or eevees, you were expected to focus on your favorite if you had one and largely ignore the others. Because these trios weren't going to be as prevalent, they were never designed to be internally balanced. They were just designed to each have valuable skills that would work in concert with whatever team you designed yourself. Grass sucked as a typing for a long time (and arguably is still significantly underrepresented in competitive battling), so it would have been hard to set up a group of legendaries with an obviously weak link Grass component. --- Well they tell you: never hit a man with a closed fist. But it is, on occasion, hilarious. DW FC: 0430-4094-0916 DW Dawn FC: 0344-7129-6960