I think money is a big part of it. Most of those sailing older boats are on them because they are more affordable. Their budgets are tighter and they are more sensitive to the cost of gas, entry fees and lodging. The economy has been tough on a lot of people. Many have lost jobs or have taken lower paying jobs.
As an example, I drive 7 hours or more each way to most regattas I attend. That's nearly $200 in gas. Then add entry fees of around $80 and lodging, which could easily be $250 (I'm usually able to stay with the few friends I have). Maybe we should make an effort to keep entry fees low and try to house out of town guests. Regattas with on site camping are very popular. Plus, it is fun to camp and hang out with the other sailors. Spring Fever has been popular because of the camping and the price break given to those driving 500 miles or more. Ocean Springs regattas are popular because of the on site camping among other reasons. Also, promotion is huge. If people expect a big turnout and everyone is talking it up, they are more likely to come.