For us it was a combination of the lack of places to visit/things to do and the unwelcoming attitude of neighbours/locals.
We're both originally from the south coast of England (Southsea and edge of the New Forest) and whilst we enjoy walking in the countryside and along the coast, we also have a keen interest in architecture and design. We bought a 400 year old detached property, approx two miles from Llandeilo that was in need of major work after being on the market for almost ten years because no-one locally was interested in saving it.
Our first interaction with our nearest neighbour went badly as he told us we wouldn't be welcomed by the local community unless we joined them in opposing a nearby wedding venue. He proceeded to make our lives hell for the next couple of years till he realised it was having zero effect and gave up. The local farmer whose land our water supply came from, cut off the supply after falling out with the same neighbour before we purchased, so we had to unexpectedly install a borehole (£11k) as one of our first jobs on the property.
Despite enjoying walking with our two dogs. who sadly passed away whilst living there, we're not particularly outdoorsy other than being keen gardeners (we created a 0.5 acre garden adjoining our property from scratch). I think if you like lots of outdoorsy stuff, Wales can be great, but we missed the sheer volume of art-related opportunities on offer in England and this was the experience of other people we know who relocated there and have since moved back/are in the process of doing so. These are all couples with either adult DC no longer at home or child-free couples, all late 40s-mid 50s. In our experience, Carmarthenshire was too far for friends/family to visit regularly (our adult DS visited just four times in six and a half years), so where we hoped to be hosting lots of weekend breaks for them, this never happened.
Additionally, we were stitched up by a tradesman to whom we paid a large deposit for heritage style timber windows - he made a couple, then disappeared. In the end we built the rest ourselves - at least we learned a new skill!
Work-wise things weren't great either - I was running an online business when we moved to Wales in 2018 and DH was a self-employed conservation builder. He found it impossible to get work except as a gardener/handyman earning £70-80 per day - and that was often questioned/begrudged. Eventually that dried up (after he was royally screwed by a local sparky) so, after being diagnosed with severe depression in late 2023 (our experience of the NHS in Wales was something that we can't fault), he worked part time in an auction house till we sold up and thankfully came home in 2024.
Wales is an undeniably beautiful country, but life there is definitely not for everyone.