There are a lot of reasons open wheel drivers are moving to NASCAR.
1. Extended driving careers - What was JPM going to do after F1, sell time shares in Miami? Why not come to NASCAR and race competitively for many more years. I think you'll see J. Villeneuve and Scott Speed in stock cars very soon (JV is testing cars and trucks for Bill Davis Racing). Paul Tracy has been on again/off again in NASCAR for the past couple years. He'll get it together. Franchitti won't be the last.
2. Sponsor leverage - The best phrase I can come up with, meaning both drivers and sponsors want to leverage the marketing opportunities NASCAR provides. Open wheel racing in N. America is struggling yet sponsors want to be involved in racing and have equity in certain drivers. If a driver can get together with their sponsor and an owner, an opening can be created. Either that or lose the sponsor (and your racing career) when they lose interest.
3. More level playing field - Blather on about NASCAR all you want but even the biggest hater has to admit NASCAR has created a reasonably level playing field for its participants. So far this year 14 different drivers have won races in Cup. I think last year it was like 17 or 19 drivers. I don't think there is a top level racing series anywhere in the world with that kind of equality. Most racing drivers want to win. Why race in a series where there's little chance of winning unless you drive for the top team?
4. Less stress/better quality of life - Racing is a grind no matter where you race. It's easy to get burned out, even if you race in some of the most exotic places anywhere. While Nextel Cup is still a grind at 36 races per year, there is no international travel. Most drivers go straight home after a race. When you spend your life living out of a suitcase year after year, things like sleeping in your own bed become extremely important. NASCAR is very family friendly and the drivers have pretty good camaraderie among them. That isn't true with every racing series. Most everyone gets along because they all know they're in it together. NASCAR simply isn't as dog-eat-dog as other racing series are. Just ask JPM, he knows all about it.
There are other reasons but those should give you an idea of the appeal of NASCAR. Money buys you a lot of things but it can't buy you lower stress, time with your friends and family, and a longer racing career.