(Off topic: I've wanted to join this forum for a while, and this was the feather that made the camel sign-up)
Well, I would say the reason that Orcs have these siege weapon because that's the focus of their culture. Dwarves mine, so they make cavernous extensive subteranean fortresses, Elves build tall spires because they reflect their natural tendencies to strive for their hight of perfection, and Humans build quickie castles (a one sided, I mean come on, whoe else makes a one sided castle )
So it stands to reason: Orcs are conquerers, so they put their advancments into the ream of Siege weaponry, so as to keep up with the forstifications of more 'enlightened' races. In other words:
To Orcs the best defence is non-existant, because they have nothing to defend, thus, their offense has a distinct advantage. You'd think this would mean they could get some halfway decent bodily protection, but I guess since the whole race is militant, it would mean that all they could afford to mass-produce, despite all of their 'loots', but maybe I'm just overanalysing things.