Don't actively collect paper money myself, so I don't know whether the Dutch are avid note collectors. But as for coins, yes, collecting them sure is pretty popular in NL. The Open Mint Day (Utrecht) has lots of visitors every year, and the Muntmanifestatie (Assen) or the Holland Coin Fair (Den Haag) attract many as well.
Why Maastricht/Valkenburg? Well, if a show has a good concept and is known for being a major venue, it is likely to attract even more dealers and visitors. Another thing to keep in mind is that, around here, you can travel from one EU country to another without really crossing a border - legally you do, but practically it's like crossing a city or state line. Now if you look at a map and see where Valkenburg is - from Belgium and Western Germany it is within easy reach, and Northern France and Luxembourg are pretty close too. (No need for a bureau de change either.) Besides, both Valkenburg and Maastricht are major tourist destinations. Maastricht I like a lot.

Christian