The driver, a native of Columbus, Ind., made the announcement during the International Motorsports Industry Show in Indianapolis on Wednesday.
"As much as I would like to do it, we just don't have the time to do it proper," Stewart said. "The IRL is so competitive now, you're not going to just show up like drivers used to do in the past and be competitive. These guys don't leave anything on the table there. You're not going to stroll into the Indy 500 with these guys who race every week and be as competitive as they are.
"It was a very humbling comment and offer that he made, for sure," said Stewart, booed by fans when he made the announcement. "I'm very humbled by the fact that he offered it to us. It's a great opportunity, but it's very hard when you're running three Cup teams right now and the obligations that we have making sure that we're doing the right things, we have a lot of people that depend on us. … It's kind of like being at Thanksgiving dinner. My plate is finally full. I don't know if I can add anymore to what I have right now." (USA Today)