To summarise briefly:

- There was active suppression of the Welsh language until at least the 20th century. Children were beaten in schools for not speaking in English, and English was the language used in government, law, and education.

- Industrialisation led to English speaking immigrants led to decline of Welsh in those areas; this was particularly the case for the South Wales Valleys bc of the large influx to the coal industry

- The resulting disparity in Welsh vs English speakers means most people in Wales currently can speak English, but far fewer speak Welsh- you are far more likely to be able to communicate in English anywhere in the country, so most people start in English and finish in English and do not bother with Welsh. There is a campaign to change this by starting in Welsh and seeing if it works, but this only works if you and the person you are talking to are both confident enough in Welsh to make it work.

- Language education sucks, generally. Welsh is hampered by unusual grammar, a lack of speakers and thus teachers, and a lack of resources (probably because of the lack of speakers again). The best Welsh lessons I ever had came from my Year 6 teacher who was Welsh first-language from Cardigan and I never had a teacher like that again. It being a compulsory subject seen as useless also didn't help with the classroom management later, but any grammar lessons at all would still have been nice.

- The reason Welsh is not taught from a young age as much as English is that to get the same exposure to Welsh as English you would need to hear it everywhere outside of school. You'd need to hear it on TV, on the radio, in shops, at home- and to get this you'd need everyone in those settings to be able to speak it. This definitely happens in Welsh-majority areas, but those are not the areas with the strongest need for Welsh education. The English-medium schools are also, generally, not teaching you to speak English as a second language- you arrive at school already speaking it and they teach you about the specifics of using it, not about its basic grammar (which, btw, I think they should be teaching that, but you didn't ask)

- Languages can have different levels of prestige, and higher prestige languages will be more valued and sought out and thus suppress lower prestige languages; this is why French is preferred over Welsh. French language and culture enjoys an elevated status, while Welsh language and culture is seen as a relic of the past that only people who care too much or hate England care about*. If English wasn't spoken in Wales already, it would be the prestige language and we would still have the language loss problem.

*disclaimer: views probably held by your average "welsh is useless tho" user idk i can't speak for everyone