When it comes to board attendance, Trials is bigger than BMX. At least here in Germany. I think the main problem is that people nowadays think that you can't do it on a MTB and that you need a special bike for it. The slow process to be able to do remotely what good riders do let's many people quit I think. If you don't have anyone to ride with this happens even faster.
This coming from someone who's been riding for 4 years.