Again, ubiquity. Especially since the vast majority of Americans who make coffee at home do so in drip coffee machines, there just isn't a lot the typical American is needs to heat up hot water for, so to most people an electric kettle is a non-mandatory item. Even most American tea drinkers honestly aren't daily tea drinkers (myself included), so for many the benefit of having extra counter space beats out the benefit of having convenient hot water, and a stovetop kettle can most easily be put away in the back of a cabinet somewhere.