I don't have hard data, but a large part of this is probably ecosystem. TypeScript updates more often and, most importantly, almost every relevant library has TypeScript type definitions available. Interesting hard data would be the percentage of popular libraries with TypeScript type definitions available vs. those with Flow definitions available. I'm quite sure that would heavily tilt towards the former, though.

The proportion of Flow users who said "would not use it" has increased substantially over the past year. (24% vs 41%)

I started using TypeScript professionally for the first time this year. A few more datapoints: I've had recruiters calling me for TypeScript projects, but not for Flow. This is in the Netherlands.

Don’t have to take the survey so seriously. It’s not representative of all JS community. My circle of JS hackers don’t even bother to participate.

You're mistaking "representative" with "inclusive".

The survey can be a representative sample, without including your ̈́"circle of JS hackers". That's how surveys work.

Is there any reason to suspect Flow users are less likely to do the survey than, say, TypeScript users?

I can speculate here a little bit. Majority of survey users has less than 10 years of experience. That makes them susceptible of being Visual Studio Code users. VSC - Microsoft product. Typescript is Microsoft product as well.

I am curious (as a VSCode user) what makes you conclude that junior devs are likely to use VSCode?

It is really good tool and free one. Better tools are either commercial or requires a lot of learning.