I've basically covered this on a podcast, so let me start with that.

Your answer is in this episode: https://www.acast.com/historyofgermany/026-frisianfreedom

That show was actually about the region south of Dithmarschen. But the cause of their freedom was the same:

It's swampy and tough economics there. There were NOT actually independent on paper. They were under the Bremen Archbishop, for example. But since that was tough to enforce, and the people so poor that they were all basically egalitarian and helped each other -- no nobility formed. Just like in East Frisia.

Added to that, it was a buffer state between Denmark and the German empire. So, just weak jurisdiction to begin with. Far away and usually not worth the trouble.

The Franks (Charlemagne) just gave up on the region after holding it for 20 years.

Technically there were under Hamburg (church-wise) ..but probably barely even knew that (in the 11the century control was just nominal on paper, really).

What you are referring to is the same that happened in other remote, coastal areas. They eventually founded a "Bauernrepublik" or Farmers (Peasant's) Republik. Meaning they elected 48 judges to run things.... which... just like in East Frisia, eventually started to come from fewer and fewer families, and the judges dynastic families became the de facto nobility.

The local "militia" were good at fighting in the Watt. They could even just open the dikes and flood the enemy.

Don't underestimate the border factor though.. Even in the 19th century they were still kinda part of the Danish crown, and part of Bismark's Prussia, until wars were fought. But just the fact that they weren't clearly and directly ruled gave them a lot of freedom. Only 1866 was it really, finally, officially part of Germany (Schlesswig Holstein Prussia).

Sorry about the lack of sources. I can update this later.