It might have something to do with gym leaders as seen here but apart from that here are some pointers.
- Some need to be more powerful in order to protect themselves
- This includes Gym Leader battles too. Lets say you are up against a Electric Gym. The grass starter would have resistance and the fire types would be neutral so there is no need to evolve. On the other hand, the water type would be vulnerable and so may need to evolve. (Otherwise things would be unfair)
- It simply may have been that GameFreak wanted it this way.
- If everything was the same, 'there wouldn't be a spicy hint' as Cilan would probably say. xD
I think it makes things a little complicated but hey ho!
Hope I helped! =]