As Vulindlela744 said, it's because they're a charter airline. More space = more pax = more dinero.
But I think there's something more to it: Have you ever tried to get somewhere on a plane, but realized it's impossible because the isle is packed with people waiting in line to go to the can? If I don't remember it wrong, there's a small waiting room down below, where the toilets are. That way, people won't have to block the isle.
That's what I think. It's a genious idea!