I think its a reality that by and large most developers are coupled to some CSS / Component framework that they aren't writing all the CSS from scratch, and most of these frameworks only (currently, when I last surveyed, say ~3 months ago) provide Flex based CSS utilities. Until recently this was particularly true of Bootstrap and Zurb Foundation which were arguably the most popular of the CSS frameworks.
I also think its general knowledge. Took a really long time for everyone to use Flexbox too. Even as late as 2016/2017 I saw engineers still reach for things like unsemantic[0] (the 960 Grid successor) and other utilities like it if they were doing custom layouts.
[0]: https://unsemantic.com/