My missus is Welsh and her number one sport is Union by far, soccer distant second then pretty much any other sport (I did get her to watch Aussie Rules games with me and she’s slowly getting into it)As a Welshman, I'd say it's much less genetics, and much more free-to-air access ensuring strong grass roots.
On SM-G955F using BigFooty.com mobile app
For most Welsh people rugby union is number one, I was told