This is a cool idea, but I don't think that there's an answer that doesn't involve some sort of story element that the players will be distracted by/try to turn to their advantage.
If you don't want it to be something the players can remove, think about having some sort of natural feature exclusive to that particular area that causes the null-magic zone. Possibilities include:
There was an intense magical battle fought in the distant past, and it was so fierce it permanently disrupted the weave of magic in that area
There are some sort of strange crystal growths in and around the town, and they have some sort of radiating aura that disrupts magic. All attempts to harvest them by traditional means have failed thus far, due to an incredibly tough structure, even adamantine has proven ineffective in so much as scratching the surface.
There's a curse on the town, but it requires something nearly unattainable for mortals in order to be broken.
It doesn't have to be something that actually exists in a rulebook somewhere, this is world building after all, you're allowed to make stuff up.