Because he can.
To be fair, Busch was placed in a position to take the shot when asked first about being compared to the elder Earnhardt and then about how his souvenir sales are picking up.
"We all know who is No. 1 and forever will be," Busch responded, referring to souvenir king Earnhardt Jr. "To me, I go out there to win races, to be No. 1 on the racetrack. That's where I feel like I win, where my benefit is.
"For me, I don't think I would enjoy having the most fans out there. I actually like the way I am, the role I portray. There's probably too much pressure on one guy's shoulders who doesn't seem to win very often."
There's the zinger. There's the shot fired across the bow of Earnhardt Nation.
But was it bragging or simply saying the facts? Busch insists he's not much of a bragger. He didn't even know who Dizzy Dean was or that he coined the phrase, "It ain't bragging if you can back it up."
But he likes the phrase. And that takes us back to why he can take a shot at NASCAR's most popular driver. Because he can.
One could argue the opposite, noting Earnhardt has 18 wins, including a Daytona 500 victory, to Busch's 14. But that's a bit skewed because Earnhardt is 34 years old and Busch is 23.
If you want to compare the two, look at their numbers since Busch was released from Hendrick Motorsports to make room for Earnhardt in 2008. They aren't even close.
Busch has 10 wins. Earnhardt has one.
Busch has 20 top-5s. Earnhardt has 10.
Busch has led 2,190 laps. Earnhardt has led 897.
The only statistic in which Earnhardt leads Busch is most popular driver awards. He won last year to give him six for his career, and he'll likely win again this year. Busch has none and won't be a candidate for one any time soon, judging by the boos he heard before and after Sunday's race.
A reader asked whether Busch takes shots at Earnhardt because he is jealous. I can't get inside Busch's head, so I won't speculate. But you can't blame him if there's resentment.
Rick Hendrick might have made the move because knew he could get Earnhardt. Or he might have done it because he simply tired of Busch's attitude, which often ticked off sponsors and caused friction within the organization. Either way, Busch lost his job at the sport's top organization. The driver who replaced him was Earnhardt, and that made Earnhardt the enemy.
That Earnhardt continues to draw more media attention than anybody in the Sprint Cup garage while Busch wins more than anybody in the garage only adds fuel to the fire.
NASCAR chairman Brian France even admits the sport benefits more when Earnhardt does well. That leads conspiracy theorists to say the governing body gives Earnhardt breaks whenever possible.
You could hear the sarcasm in Busch's voice on Lap 320 of Sunday's race when Earnhardt received the "lucky dog" because he was the first car a lap down when the mysterious debris caution came out, so he got to start on the lead lap afterward.
There was a tad of sarcasm last season after a late run-in between the two for the lead at Richmond left Earnhardt 15th and Busch second.
"No, there's not been any death threats," Busch said a few days later. "That would be ludicrous, I guess. Maybe they're out there. I haven't seen them. I hope my well-being is safe in this deal. Since Junior ended up OK, then hopefully I can end up OK."
Ah, you have to love it.
Busch couldn't say those things -- OK, maybe he could -- if he weren't winning so often. How can Earnhardt respond? Hey, dude, I sell more T-shirts than you.
Earnhardt's lone edge is in popularity. He owns about 30 percent of the market in souvenir sales. Nobody can touch him.
Because of that popularity, if you're an owner starting a new team, you'd have to weigh the benefits of attracting sponsors and other monies against winning races.
If you're Busch, only one thing matters -- winning.
Right now, he owns that market.
Or, if you want to take a shot, he owns Earnhardt.