I think this is the largest reason right here. People are naturally going to reserve their limited counter space for the stuff they use daily. For Americans, that's more likely to be some kind of coffee maker than an electric kettle.
Growing up where I did, I knew a lot of families that regularly made iced tea. But they usually made a gallon at a time, once or twice a week, and still drank coffee every day - so they had counter top coffee makers, and stovetop kettles that could be stored away the rest of the week.
Unfortunately for every tea drinker in an American hotel, most coffee makers (at least the drip kind) will make any water boiled inside taste like coffee, unless they've been used exclusively for plain boiled water. Maybe a combo tea/coffee drinker wouldn't mind, but I've always found it intolerable.
But it's a good point about the grid - we have plenty of appliances for coffee that are principally glorified water boilers, and there's no evidence that our appliance voltage has hampered their popularity at all.