Main reason is that gnbs use slightly different gunblades to squall. Squalls gunblades are all 2 handed longswords which are rather big. Our gunblades by comparison are more 1 handed blades/ 1.5 bastard sword kind of deals and are a fair deal smaller, almost like a halfway between a Seifer and a Squall gunblade. Holding our gunblades permantly in 2 hands would look a bit strange on most of the gunblades we have, like holding a knife in 2 hands sorta deal. Now holding our gunblades in the garlean style like gaius, now I'd be down for that