The reason why potatoes isn't kitnius is because it wasn't out there when they made the kitniyut rule, but did they have corn on that time so it should be kitnius?
From OU kosher:
Iggeros Moshe explains that the minhag to not eat kitniyot developed differently than other minhagim and therefore rules that only foods which we know were specifically included in the minhag are forbidden. [See also Chok Yaakov 453:9 who makes a similar point].
With this he explains the generally accepted custom to not consider potatoes to be kitniyot even though logically they should be, as follows: the minhag of kitniyot can be dated back at least until Maharil, who died in 1427, and potatoes didn’t come to Europe until the 16th century, so potatoes were a “new” vegetable which wasn’t included in the minhag.
An important “exception” to the aforementioned rule that “new” vegetables aren’t included in the minhag, is corn/maize which Mishnah Berurah 453:4 and others rule is kitniyot even though it was introduced to Europe after the minhag had already begun.
More info on kitniyot can be found at the link above.