(but of course, the reason why CH just arrested him now can't really be what they tell. I mean a technicality like "we didn't know that he was in Switzerland before when he stayd here for holidays for several weeks, but this time, we knew he was coming", I'm sorry, that's ridiculous...)
Probably because they had a fresh arrest warrant supplied with a time and place to arrest him. The police cannot just ignore that. It's one thing to not actively search after a suspect mentioned in 30 year old, dust-catching files, one can always claim ignorance or lack of resources or whatever.
But ignoring a fresh warrant that just requires sending an officer with handcuffs to the airport would be a bit hard to justify without undermining the whole extradition treaty.