Because it wasn't Applejack's destiny to stay there. If it weren't for Rainbow's Rainboom, she would've stayed there and never realize who she truly is, being something she isn't. Applebloom on the other hand, her destiny is yet to come. If she gets a cutie mark that's unrelated to apples, if it helps on the farm she'll stay, if not, she'll take her destiny elsewhere because that's who she is and what she is meant to do. Not saying she has to leave the farm for good.
But a non-apple cutie mark can still be useful. But like I said, it depends. I'm just bringing up the possibilities that could happen.