Well I suppose its what concept fits to what idea's he has at the time and how he wants to portray his music.I don't think its fashion,or genius just mere experimentation.
You can see how media and music has changed by the increasing amount of people using the internet for musical applications,your audience becomes wider but the concept becomes more isolated,so there is less need for big concepts if you can experience the same event in your Living room.In the 80s it was not possible to share a global web concert unless it was via your TV,but even those audiences were limited due to broadcast,so it was more idealistic to share a large scale concert by going to the event,and besides Jarre's grandiose events were always connected with some form of special event or anniversary rather than actually for promotional purposes like a Tour concept
Jarre could have easily done the Oxygene Tour in the 70s,with the same philosophy,but it obviously wasn't how he wanted to connect the audience with his Music,and didn't see the logistical point in it,prefering the idea of One offs,but the concept of these intimate concerts wouldn't fit the same scale as Houston or Docklands,his music is universal in both events but just think the intention of it makes the differences,and that is down to his own perception of what the Music is trying to say