These states were relatives small and had majority German or at least Germanic speaking populations. They had historically been part of the Holy Roman Empire. Why didn't the join the German Confederation when so many other similar states did?
For that matter, did any other state consider joining?
The Netherlands/United Provinces had been a significant political and economic player in Europe, and indeed globally, for centuries prior to the formation of the German Confederation in 1815, and though their power had declined in recent decades they had a well established independent identity by this time with a unique culture and political life. Moreover, seeing themselves as an independent (if no longer great) power in their own right they had little desire to join a confederacy dominated by two Great Powers, Prussia and Austria. The Austrian connection was no-doubt particularly troubling, as in the early modern period the Netherlands had fought a long and costly war for independence against Spain, then ruled by a branch of the Austrian Habsburg Dynasty. Lastly, as a result of its maritime heritage and empire, the Dutch saw themselves as more a part of the North Sea, Atlantic, and imperial worlds than the Germanic world, having more in common with other Western European maritime nations (in their eyes especially the UK, who despite differences and several wars also had a shared history of anti-French, anti-Spanish, and anti-Catholic sentiments and early modern Reformed Protestantism) than the largely landlocked, Central European, and broadly split Lutheran and Catholic German Confederacy.
The Netherlands didn't and still don't have a significant German population, nor a significant number of German speakers. The official language is Dutch and only the Frisian dialect is recognised as a minority language. The Netherlands joining the Confederation would also offset the Balance of Power which the low countries were meant to maintain.
However after the Belgian revolt and subsequent separation from The Netherlands, the duchies of Luxembourg and Limburg were created in 1839 as part of the Dutch Kingdom, of which Limburg was a province of The Netherlands and Luxembourg an autonomous region sovereign nation (it was autonomous before that, and the Dutch King remained head of state). Both duchies did join the German Confederation.
It might not be the answer you're looking for, but remember that Switzerland is a confederation itself. Some of its cantons (states) have fought for their independence from the Austrian empire and I doubt they were interested in giving up their freedom. Then Napoleon rolled over Switzerland as well and everybody was happy when it was over.