The best bus route in Europe is arguably the 300 between Haarlem and Amsterdam Zuidoost. This 40 km route has its own lane or own road for almost the entire route. Some of the route goes on the motorway, but it is allowed on the hard shoulder during traffic jams. For most of the route, a special road was built for it, incorporating many expensive bridges and tunnels. It gets priority at nearly all junctions, and there are level crossing barriers at most intersecting roads and cycle paths.
So, given this spectacular investment, you would expect them to choose the best possible vehicle. Double deckers are not common outside the UK and former colonies, but if a double decker was appropriate, they could surely have used them. I think the tunnels are high enough, and if they aren't, they could have built them high enough to fit double deckers in. Needless to say, they use artics.
Another question is why don't we have double decker trams, if indeed double deckers should be preferred to artics if we have a choice? Blackpool obviously has old double decker trams but when the network was modernised recently, they still chose articulated trams.
Double deckers make more sense for trains, because the increased dwell times are not so important as stops are less frequent than for local buses, and dwell times can be reduced by having very big doors. That is why double decker trains are common in many European countries.