Because traditionally Stormboyz were young Orks with a discipline problem (i.e. they had it).
Thus a Warboss, being much bigger, older and less mature would never dream of strapping himself to a rokkit like a foolish young Stormboy. He'd much rather plant his feet on the ground and smash someone's face in with a big axe then do something as technical as fly around on a jetpack!
~Andromidius
I would disagree with this on only one note: Blood Axes. It wouldn't be too far of a stretch for a Blood Axe Warboss to have a disciplined mentality, especially considering the "un-orky" ideas they get from Humans. Besides, he doesn't have to be a "stormboy Warboss", he'd just be a Warboss who thought it'd be a good idea to strap a rocket on his back to get to grips with his enemies faster.
Per Codex rules, yes it is illegal, but this is what Apocalypse is for. Make up a datasheet for a Stormboy warband, or something along those lines. And while you're at it, make Big Meks on Deffkoptas, because that does sound incredibly awesome.
And even if you don't do Apocalypse, and you want to have a Warboss with a Rokkit Pack, just make one! Even if it's an "illegal" unit, it still sounds like an amazing idea for a conversion (along with the Big Mek Deffkopta), and it will just add to your collection. Kinda makes me wanna make something like that.