Steven,
I think your numbers don't tell the whole story. In 2001, Reno was still "exciting". The National Bowling Stadium (NBS) was still a spectacle. It was only the 3rd time ever hosting the tournament. Reno, ever since 1995 when the NBS opened always saw a dramatic increase in entries. Let's look at a better picture:
2009, Las Vegas, Nev., 17,200
2008, Albuquerque, N.M., 12,615
2007, Reno, Nev., 16,235
2006, Corpus Christi, Texas, 12,606
2005, Baton Rouge, La., 13,222
2004, Reno, Nev., 16,585
2003, Knoxville, Tenn., 12,203
2002, Billings, Mont., 10,806
2001, Reno, Nev., 16,104
2000, Albuquerque, N.M., 10,688
1999, Syracuse, N.Y., 9,912
1998, Reno, Nev., 15,925
1997, Huntsville, Ala., 9,480
1996, Salt Lake City, Utah, 9,764
1995, Reno, Nev., 17,285
While I personally enjoyed Billings, played my very first game of Texas Hold 'em there (and got fleeced by the local establishment I believe, lol) I think this picture shows that Billings did stunt the growth slightly that year. Discounting the "Reno effect" (which at this time was a positive unlike the negative version we see now) you see a 7% growth from Syracuse to Albuquerque, but only a 1% growth in Billings. Then we see bigger increases leading up to the huge numbers in Vegas in 2009. I feel this was due to the fact that it was the first time in Vegas in a long time, and it provided people with a destination that offers a wide variety of entertainment options for the first time as well.
The other thing to consider is travel. How much does it cost to go to Vegas versus Billings, versus Green Bay? Many airlines run deals to Vegas where you can find airfare at rock-bottom prices. How many offer specials to Billings, to Green-Bay? My guess would be none. In the example of the USBC Senior Masters here, why would some of the fringe and lower level bowlers spend the extra dollars to fly to Green Bay instead of Vegas. Think about it from the perspective of the guys fighting for the last cash spot...go to Vegas on a cheap flight, bowl, enjoy entertainment even if I don't bowl well or...go to Green Bay, pay for an expensive flight, perception is there is nothing to do other then bowl, so if I bowl bad, not only am I out more money, I won't get to enjoy my time there (whether it's true or not). Now add in the fact that the PBA50 scene at the moment, has many, many of the PBA's greatest players active in it right now....just look at the final 5 from this tournament. Why spend the extra dollars to fly to Green Bay, to have an even smaller chance of doing well then in prior years with all the 50 year old HOF quality bowlers?
I think the USBC needs to look at places where there are many direct flights, where airlines may offer discounted fares, to hold these tournaments. It doesn't have to be Vegas, I am sure there are other cities most of the major airlines offer discounted flight to, and then look to find venues that can host events like this. I think you will still see decline over places like Vegas, but it may not be as severe.
JMHO