From the routes online article, this sentence stands out:
"Latest figures for September 2012 show an 18.4 per cent increase over the same month last year"
It's not that big an increase in passengers if you look at what the increase in capacity is even though there's hardly any competition for them at GLA. Having made the jump to a notional daily 77W at NCL but seeing lowish loads, it may be some considerable time before they go down the route of adding frequency to NCL