I've not been to Belle Vue for a fair few years, but last time I saw it on Sky (which I admit may well influence the level of attendance) I wouldn't say that their crowds were all that healthy.
United and Wednesday probably don't attract the same levels of support, but then how many Man City and United fans actually come from Manchester?!? I would imagine that the % of Sheffield football supporters that travel to home games is much, much lower.
I think speedway support across the country has dwindled regardless of track location, the ones such as Newport and Stoke that had less support in the first place have either disappeared or had to downsize the operation in order to survive. Its a vicious circle, money makes money - eg if you make loads of money you can afford to advertise on tv and/or radio and newspapers etc,and hopefully generate interest and boost attendance/profit, if you don't have loads you can't afford to advertise and once your support starts to slump you can do less about it