There are many, many reasons why Cup drivers run in the Busch Series and you have to eliminate most of them before most of the Cup drivers stop running both series.
1. Drivers love to race!
People complain about Dale Jr. and Jeff Gordon and a few others about only running Cup races and not enough other races. The day a driver doesn't want to race is the day he needs to consider retirement.
Are you more inclined to watch a Busch or Truck race if your favorite Cup driver is participating? I think that 99% would say they'd be more inclined to watch.
1A. Money, Trophies.
Drivers want to race, but they race for that trophy. How many drivers said that they wanted that Gibson Guitar last week at Nashville? Plus the money. It's not Cup money, but hey, it's cash nonetheless.
2. Sponsors.
Sponsors who are paying multi-million dollar sponsorships want their car to run up front. They don't want to pay all this money and see they're car finish 25th every week. How do you make sure you're car is in the top 10? Put a Cup driver in it. You have that much better of a chance to get a good finish, plus, usually, they're affiliatted with a Cup team, so you get the resources, AND you have name recognition. Like I said before, fans are more likely to watch a Busch race if their favorite is racing.
3. Car owners.
Much like the sponsors, there's more money involved. If you get a sponsor involved with your Cup driver, a lot of times they'll become an affiliate in their Cup program. Also, it's more seat time for their drivers to earn experience at that track and learn for the Sunday race.
4. Track owners.
It sure as hell benefits the track owners to see Cup drivers in the Busch Series. They can sell more tickets. Like above, you have name recognition.
5. NASCAR.
A. NASCAR has a system in place that supports Buschwhackers. You have 27 of 35 Busch races that run at the same track as the Cup Series (includes Indianapolis). That makes it convienent for the Cup drivers to race both.
B. Both series run on the same tires. This is a HUGE benefit to learning something for Sunday, more so than anything else. Change the tires for one of the series, decrease the advantage.
C. The cars are too much alike. One thing NASCAR is doing that is good for eliminating Buschwhacking is the Car of Tomorrow. If the cars are completely different, that further separates the advantage of racing both series. There's talk in a few years that NASCAR will change the Busch cars further, possibly to sports cars.
Well, that's all I have for right now. Not bad for putting this together before noon... If you can think of other reasons, I'll add them.