"I used to go to Kings Lynn 5/6 times a season when they were Premier league, but don't bother these days as I can see them on the box". (Spanner47)
This seems like a pretty good answer to your question to me Spanner.
You prefer to watch them "on the Box". Perhaps that is at least one of the reasons support is dropping. Once you stop going it is very easy to find something else to do. As you say - "unless we find out, we can't begin to address the problem". You are right about that - I would respectfully suggest that if YOU prefer watch your Speedway "on the Box" so do lots of other people.
The answer, of course, would be NO League Speedway on Television and, hopefully, that would stop the Sport haemorrhaging Supporters and possibly encourage more, like yourself, to return to the various Tracks....