Ubuntu 16.04 LTS stands for Long Term Support, so it stays supported longer than non-LTS versions. Software can be out of date, but its rock solid. Instead of updating software (like on Windows and macOS) they backport security & reliability fixes from upstream.

Some people also don't wanna run 57 (yet) because their favourite extension(s) aren't ported from <= 56 yet. So they stay on older versions. Current ESR (Extended Release, akin to LTS) is 52.