Here's why:

* Prokaryotic cells are simple cells that lack a true nucleus and other membrane-bound organelles. Their DNA is located in a region called the nucleoid, but it's not enclosed within a membrane.

* Eukaryotic cells, on the other hand, have a true nucleus enclosed by a nuclear membrane and other membrane-bound organelles like mitochondria, Golgi apparatus, endoplasmic reticulum, etc.

Therefore, prokaryotes are the only type of cells that do not have a membrane around their organelles and nucleus.