I think partly social pressure, in the sense that the old school trans community was very small and there was the risk of losing most of your social circle if you stepped out of line. I actually think there were more quiet sceptics than most people assume, but because they stayed quiet nobody knew how much scepticism there was.
And partly the assumption that transactivism was sweeping all before it, the terfs were a small and declining obstable, and they'd benefit from politicians and cultural figures and business leaders all wanting to be on the right side of history. That's what Stonewall told them after all.
And partly the assumption that growing numbers, especially in the younger demographic, meant safety in numbers and more social clout. They didn't think about bad actors or social contagion.
So their assumptions turned out to be faulty.
It doesn't take an enormous amount of courage to do a Debbie Hayton and say "these extremist loonies are undermining the social acceptance I rely on", but it takes a certain amount of courage, and I think lots will have seen that Hayton is considered the Great Coconut in trans circles and thought, no, I'm not bringing that on myself.