Same answer as with most new products: Apple apparently has only a limited (large, but limited) number available at the moment and has opted to address its biggest markets first. Hopefully you in the Netherlands and other great countries will get some before Christmas, but yeah as I'm in Canada I've gotten used to occasionally not being in the first clutch of countries to get the latest hotness.
The actual reason is that the HomePod (Regular and Mini) don’t support Dutch as Siri language Apple therefore does not sell both in the Netherlands and Belgium. You can buy them over the border in Germany or France. Guess you would also be able to have them shipped to the Netherlands via a German reseller (less)
