Most likely because American politics frequently pound the talking point of "far left politics" when talking about the political opposition (moderate left at best from an overseas PoV), to the point where American liberals have been conditioned to assume that they are being spoken down to when this type of language is in play. American leftists are also very anti-authoritarian on average and do not appreciate being lumped into the same category as tankies by simple virtue of people only discussing left versus right.