I dont think you can put the blame on any one thing for dropping tournament entries. Actually, no, I lie, it does come down to one thing:
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$
That's right, plain for everyone to see, it's not just the airfares, or petrol money, or whatever means of transport used, it comes down to the money. Personally, even though I'm just a junior, I cant justify spending up to $500 on transport, accomodation & entry fees, when in some cases $500 can be more than 1st place pays out. And even if I was confident I was going to get back more than that $500, I'm damn sure I would have trouble acquiring that sort of money in the first place. Then, you will probably find that budding tournament bowlers that cant afford tournaments became stagnant & complacent with league bowling, & eventually drop out of the sport altogether.
I also think bfcc has a valid point. Why fork out $1000 to attend a tournament that you're really just donating your money to the big boys? When you can stay in the comfort of your home centre & bowl for money you'll have a chance at winning. I know Heritage Bay Pty. Ltd. aka Olympic Sporting Supplies has been running a series of "Olympic Star" leagues in which you can win $400rrp worth of gear or $192 cash for bowling 18 games over 6 weeks. At $20 a week, why not?
So, really, Airfares aren't killing bowling, the almighty dollar is.