USBC Open participation is declining for a number of reasons:
1) Reno burnout - While I understand why the tournament was here so often, many are tired of coming to the same city over and over again. But, those who think it was expensive to fly to Reno, will get a shock, I think, when they book tickets to El Paso next year.....hint, it ain't much cheaper, if at all, and the hotel rooms will be more expensive then the casino rates in Reno.
2) US economy - The Economy as a whole is no where near where it was pre-2008. People are unemployed still or underemployed if they are working. Luxuries like going to the USBC Open are just not fiscally possible. Especially going to the places we go which usually are more expensive flights (as they aren't high traffic destinations).
3) USBC Membership - As avabob mentioned, USBC Membership is declining. Some of this is the fact that the youth bowlers have found other things to occupy their time, soccer, video games, etc. We don't have enough young bowlers coming in to replace the older bowlers who are leaving the game. Add that to the fact that a lot of leagues are bowling unsanctioned now. And if that was your only league, then you have no USBC average. Would a 160-170 average bowler go to the USBC Open now, knowing you would have to compete in the Open Division? Heck no. So again we lose entries.
There is no easy fix for this. The USBC needs Reno for this tournament....it offers the cheapest and easiest way to hold it thanks to the NBS. Add in the fact that the number of cities that have a convention center big enough to hold this event, that would be willing to turn away yearly business for the time needed to hold it, is dwindling and you wind up in Reno more times than not. Now add in the fact that due to the lack of participation (and higher then normal scoring) most people aren't getting back the amount of money they are used to, and you have a downward spiral feeding upon itself. Many will now not come back due to this, which will make it worse next year....and so on.