Its because a greater proportion of younger adults already have some GCSEs so there isn't the same demand.
There is probably an element of that, bit it is also funding. In the 1990s/2000s, I took Spanish and German from scratch through GCSE to AS-level (there weren't enough people in either case for yhem to run an A2 class,) There just aren't even beginners classes these days. There are some conversation groups for people who already have the language, run by volunteers.
I started Welsh in my English town - I'm continuing it online with a Welsh college - partly because the Welsh government funds Welsh education. People who are teachers in Wales get the courses for work if they need it, as do other people in some jobs in Wales. (I mostly do it for fun.)
In the past, I did an OU module for general interest; since they had to offer tuition fees at the same rate as other unis, that's not an affordable option, either.
The local uni does some extra mural courses, and there's the WEA, though mostly only during the day, which isn't helpful for those of us who work normal office hours.
Part of me would like to do maths A-level, but just for interest, not work or anything. I work full-time, so evenings are the only option. Can't afford the OU, I don't see how I could do it currently. 20 years ago, I probably could have.