I think the opening line of your post sums it up perfectly.
Birmingham as a city has changed beyond recognition since the halcyon days of the 70s and early 80s, especially the demographic, which really is the key here. The 21 year gap between closing at the Wheels and the reopening at PB is another major factor. Add to that the fact that speedway is pretty much a minor sport now and you can see why the club is struggling to survive.
Oh yes, of course, there are also plenty of other ways to get your sporting fix, as has been pointed out in previous posts.
TBF, it's done well to last as long as it has.