the weird thing about Sevilla which is different than Real Madrid is that Sevilla were playing terribly in their league and they managed to win the Europa League, in the other hand Real Madrid were still quite good in La Liga and they won the CL, so it wasn't as surprising as Sevilla, seems like spanish teams have some extra power on european leagues.
It's the Spanish teams play. The go for players that fit into their style of play. Spanish teams are very technical when they play. A Spanish team can be playing defensively against a better opponent but they will be very close to the half way line and very far from their 28 yard box. The don't believe in strength and pace. They believe in taking their time and build up play up to the moment they will score.
If Sevilla player that final against a Spanish team, they would most likely not have won.
Madrid can be struggling in the league and still be balling out in Europe. It's only Barcelona that needs to doing well to win in Europe. This is because Barcelona don't know how to win if they don't play well. It's just this season that Xavi was able to squeeze wins out of Barcelona even if they don't play well.