Kengle wanted to make his own Kiwi Farms with blackjack and hookers. So when the time came to pick a fruit/vegetable name, he could've picked onion instead of kiwi because lol so random, or some other Kenglish reason like cat said.
Even if Kengle denies trying to make an imitation KF, there's the fact that he chose the admin name "OnionNull" on the former forums, he set up Onion Farms on a .ms domain (like KF was), and he moved Onion Farms forums to a .net domain (like KF did).
That, or he really lacks the originality to run a site differently. Kengle is the kind of person who thinks up names like "Africanople," "Candiceport," or "Lumberton" for a supposedly exotic "fantasy" setting, after all.