I don't think it's a matter of EZY or LS being too busy competing with TOM/TCX. The fact that both have tried and now have little or no city break destinations suggests that from NCL there is little or no money to be made from euro city breaks. However, BCN, GVA and NCE are popular weekend destinations.
FR couldn't make 'Milan' or 'Oslo' work neither.
Besides, by the time you faffed on going to the airport, checking in, security, shopping and boarding you might as well just sit on a plane for an extra 30-60 minutes and fly to Spain.
AGP is my go to place for a weekend away. The fact that this route (and ALC, FAO) is served so regularly and year round suggests to me that I'm not the only one!