If I am not mistaken, the chipsets on the Intel NUC models are laptop chipsets. I have the Skylake Core i3 version and that supports up to 32G of LPDDR4 RAM (dual channel). Since the thin laptops typically have only one RAM slot, it makes sense why they are limited to only 16G.

With this in mind, my understanding is that it is perhaps a limitation of the current Intel chips for single RAM slots, combined with the reluctance of the laptop manufacturer to include dual channel memory in their (thin) laptops.