I noticed that most Flemish/Walloon nomination dossiers are made up in French. I assume this is done because English and French are the working languages of UNESCO and Flanders has some experience with French correspondance (in dealings with Wallonia and so on). I assume the name Béguinages originated here (as I have no idea what the English word is, or if there even exists one)
That being said, I always based myself on this
map I found once. I believe it was made by an university student and shows all the beguinages in belgium. There are indeed beguinages in Wallonia, but they are incomplete, partly preserved, not intact as an ensemble. I know that Hasselt and Antwerpen for instance have beguinages, but they are incomplete and therefore deferred as world heritage. I assume this is what happened to the wallonian beguinages, I always assumed there weren't any fully preserved ones in Wallonia.
If there is an intact one, naturally it should be included. Plus the movement started in Wallonia.
There are also as far as I know 2 intact beguinages in Holland: Breda and Amsterdam. I wonder why those weren't included (though it could have something to do with the thematic choices of the dutch government, and maybe religion as the movement has catholic roots)
However I suspect the Flemish region wanting to have a world heritage site exclusive to Flanders, something typically Flemish, a way to showcase Flemish culture. Flanders always has had an underdogfeeling about it's own culture and I think they were very glad to have something of which they could say "something you can only find in Flanders". "Something Flemish is world heritage worthy". I must admit as a Flemish I always liked this sentiment of having a world heritage site typical to the region, so I might be a bit partisan to not extending the list. But putting my heart aside, logic dictates that other worthy beguinages should be included.
I also just read the criterion on the unesco site and one of it lists:
"Criterion ii: The Flemish béguinages demonstrate outstanding physical characteristics of urban and rural planning and a combination of religious and traditional architecture in styles
specific to the Flemish cultural region."
Notice the last line. Maybe one of the reasons also why the site never got extended.