I eat peanuts on Pesach myself (or would if I had any inclination to eat peanuts). But corn has been Nahug for 500 years not to eat. It started for a reason; it wasn’t a mistake. It might be a later Minhag, but it is just as strong a Minhag, even if the reason is just a quirk of history.
And you can’t be Matir Neder to not eat Kitniyos. Real Minhagim don’t work like that.