I can only speak for $MY_JOB, but I'm pretty sure everyone was on Atom before VSC "got good". Atom had a good plugin ecosystem; what really drove the change was Atom's horrible performance issues whereas VSC was snappy and responsive.

What I believe also influenced the shift was that at that point in time MS had accumulated a decent amount of developer trust by giving us TypeScript and later on by acquiring GitHub. They appeared to care and have the right vision for open source.