BMX'ers need places to ride, as their sport cannot be done on stuff that is all ready there.
Trials riders don't need a special place, i live in a small town and there a gigantic range of things to ride. Steps, rocks, walls, rails. But there is never any 'ramps' or jumps laying about for bmx'ers.
Also, a trials park dosnt realy need any money investment. All you need is an area of land. Then you can normaly get creats and those cable reels free of charge, if you explain what they'll be used for. Then all you ned is man power to get it to the spot.
I just don't see the need, its the same princeipel of building a running track. Where you can run just about anywhere, but people pay for the land, and to have it all marked out.