Linux

Because it's a random set and spread across the whole hundred million users of Steam. The chances of any person being selected more than annually are low.

Conversely, I game only on a Linux machine and have received 3-5 surveys in the last year.

It may have a variety of factors, but it is not as simple as "they aren't giving surveys to Linux users to deflate numbers" or something that some Linux users tend to theorize when they don't get surveys for a while.

I got it only 2 times in the 2 years that I've used linux, before that not even once for 15 years

You can always deny the survey...I would hope that means they don't use the data anyway. But I understand the motivation if they do (not a truly random sample if people are more likely to deny on one platform vs another).