Airlines buy slots at airports to allow them to land, turnround and take-off, all these are at a premium and there are only a limted amount. The better slots, such as the Sunday ones will be bought up by the scheduled carriers as they make more money on the route. Charter airlines fight over what is left, Corfu is an extra pain as arrivals are limited and approaches dfficult.
As for Glasgow or Newcastle I would go with NCL every time. Bright modern airport with easy access, the same cannot be said for GLA. Also punctuality is infinitely better at NCL as they have the facilities to handle the aircraft. Your passengers will be happier with Newcastle.