Because they don't want fingers pointed at them, just in case they want to do it too.

The US desperately needs a grass-roots movement that lies low and makes do with harm reduction so the ones in power do not see it coming, and then suddenly switches their vote together all at once when they know they have enough numbers for a third party to surpass the other two, and be able to pass constitutional amendments, fix the electoral system, and other changes to start undoing the damage of McCarthyism.