Why is the constructivist theory applicable in teaching mathematics?

My answer to a question on Quora: Why is the constructivist theory applicable in teaching mathematics?

I suggest to modify the question a bit:

Why do some people find the constructivist theory applicable in teaching mathematics?

This wold allow me to express my surprise: indeed, why?

I have lived in the world of professional research mathematics for almost 50 years now, and I wonder why constructivist theory in mathematics education so blatantly ignores the experience accumulated in the mathematics research community. I feel that the  constructivist theory talks about some different kind of mathematics, not the one known to me and my many friends and colleagues from all around the world. But I am Vygotskian by my philosophy upbringing, and I can see how Vygotsky’s sociocultural approach explains the invention of this mock image of mathematics. I will look for an opportunity to explain that- I hope Quora sooner or later will give me chance to do that.