I think there are a number of factors.
There are still plenty of people that are into DH but the quality of long travel trail bikes has probably reduced the number of people at the less serious end as they are so much less restricted in terms of access of where and what you can ride and have fun on.
Another issue is probably the entry requirements of the BDS, while its good to have the premier series in the country more elite focused, it is at the expense of a lot of the people who make up and support the scene. These are the people who actually pay for their own entries and kit and also come to enjoy the event.
The fact that there are no longer UCI points available, means that there also also no incentive for mid pack elites to be there either, if there is a field stacked full of top World Cup riders competing, then they have no chance of getting the points they need to race a World Cup. They are better off going abroad to race and at least get the chance of riding in some decent weather.
The BDS has had some bad luck with weather conditions, especially at some of the Scottish races, which has had an effect, with people not keen on driving all the way to Fort William to get hardly any runs.
There has been a slight downturn in popularity overall and even the Pearce series which sold out in 3 minutes two years ago has entries available for the last three rounds.
The (BDS) races are expensive but with the ever increasing amount of safety regs and requirements that the riders see as essential, I don’t see how they could be any cheaper. Riders always want more uplift vehicles, more bike washes etc, etc but never want to pay any more for them.
I personally think that they have done a great job recently and provided great events at a reasonable value for money.