[SEN has too few onward connections]
I don't think that's a major factor in this case. The good burgers of Kristiansand and its environs just want to get to London as swiftly, and probably as cheaply, as possible. SEN offers that - it's a straight-in approach without worrying about the delays in London airspace, swiftly through the terminal, 30 seconds to the railway station and 53 minutes into Liverpool Street.