Maybe SDA is less popular as the BDS has gotten a bit better?
So popular that for the last few years they have to open up the entries with anyone with a licence just to try and fill them? Numbers are falling, the bigger race series' (other than Pearce) have all struggled to fill up. There is virtually no racing in South Wales these days, other than Taff Buggy.
The little local races are good though, ~£20 a pop, only one day and they can get away with only 50-75 people showing up, whereas the big races needed 200+ at a minimum to work.
I rode with a big group of people who raced the BDS a few years ago, probably 10-15 of us, none have a DH bike any more and most race enduro instead now.
Me, I got sick of spending £65 on a race entry to get my bike wrecked on various sh*tty uplifts, to spend 30 minutes on it over a weekend if I was lucky.
I still race the local events, but that's my lot. The difference is, a lot more people use trail bikes at them.