What did they do to make everyone lactose intolerant? And why is that such a thing now?

That has two causes that I am aware of. First, some people are genuinely lactose intolerant due to a gene variant in some ethinic groups. Second, pastuerization. The heat destroys lactase, the enzyme that the body uses to help digest dairy.

Pasteurized milk is a big factor. Raw milk contains every enzym that's needed to digest it well. iirc @Max DeMarco did a video about that topic