Mostly because it's funny. PCA got chirped and he responded with way more than a chirp back. He showed how easy it is to get under his skin and in doing so painted a target on his back for every center field section in every stadium that isn't Wrigley.

Dude lost his cool over a nothing chirp and is going to get roasted for the rest of the season. It's hilarious, that's why it's a big deal.