Think age is a factor.
During early 90's we had a young fan base and I reckon 80% or so we're under the age of 30. As the fan base has gotten older, the younger generation haven't replaced us and when you look at the crowds nowadays I'd say that 80% are over 30.
As our aging fan base gets older people have moved away, gotten families etc.
The cost of tickets on a match to match basis takes the piss if you have a family, and despite what people think, walk up fans are practically non existent due to the fact you have to queue twice to get in the ground.
For the 15 years or do we steadily grew our core fan base with promotion, and cup runs despite the riddler and scams antics, then the last 3 or 4 years the club decided to do its best to ruin everything. We seem to have turned a corner and things are improving, but I think we need something like a good cup run to get the lost fans/youngsters to start coming back.