I have some personal, subjective opinions about why this is such a problem in the US.
Namely, if you consider it from this perspective:
In the past 60 years, the US has gone from institutionalized segregation to gay marriage.
you can see how much the cultural landscape of the US has been turned on its head in just one human lifetime. The more rapid the cultural changes, the greater the culture shock, and the greater proportion of the public that will exhibit some form of negative emotional/behavioral consequence. Don't get me wrong, moving in a more progressive direction is a good thing. But doing it at a break-neck "Fuck all those backwards hicks" modality is only going to radicalize the people that cannot adapt to the changing cultural landscape.
If done at a slower pace, these people (domestic terrorists doing things to a clinic that just hands out condoms and does discount ob-gyn services) would die of old age before they were pushed into radicalization, and the transition would be smoother and with significantly less violence.
The problem I see is that modern progressives see any slowdown as an existential threat to such inevitable progress, and consider "going slower" to be morally unthinkable-- completely discounting, if not outright ignoring the consequences of such rapid social change on an aging population demographic.
Fun activity: Check the ages of most of these terrorists. I doubt that many will be in the 20 somethings, and that the median will sprawl around age 50.