Ticket pricing is problem here, too, and I don't think companies address it very well or consistently. As someone who has been convinced that only by paying full price can I truly be "supporting the arts," I admit to feeling somewhat annoyed to find seats being filled by last-minute giveaways and by papering the house.
Perhaps the most overlooked reason is society as whole these days. People (again all of my comments are merely my thoughts on the matter and by no means definative) seem to derive more pleasure these days and only seem to be able to make themselves feel better when they are putting someone else down for any reason they can find. One can hardly scream at a soloist to "get a move on" like at the football, or cricket for example, whilst eating a pie and drinking a beer. Ballet requires discipline not do, to watch and from that comes enjoyment (for the dancer and the audience). The irony is whilst the "older" population becomes more and more undisciplined and young girls are often criticised for not being disciplined and having no direction, in a small regional area of tasmania there are more than five ballet schools all full with young girls wanting to learn the art of ballet.
It's sad whent he requiement to sit quietly and pay attention becomes a deterrent to attending any performance. But I think you're right. There are so many "audience participation" spectator sports -- movie attendance being one of the most prominent nowadays -- that people may have to feel the it's downright odd to take part in what is seen as ancient and out-of-date 19th-century audience rituals.