I might be mistaken but i think you can move and shoot with it too.
Rule section for
SAG states specifically it is treated as a heavy weapon for moving and assaulting purposes.
I'm not an ork expert but I think you see the
KFF more right now cause in 4ed it is a fantastic boost to most ork armies. Since most ork lists run a warboss in one slot, there's only one slot left and that is invariably going to a
KFF.
5ed may change that a bit, since the biker warboss is harder to hide and units can gain 4+ cover via various means you may see
SAGs more often. Although 5ed the
SAG will scatter alot more, which may turn people off to it.