Here's why:

* Eukaryotic cells are complex cells that have a membrane-bound nucleus containing their genetic material (DNA). This is a defining feature of eukaryotic organisms, which include plants, animals, fungi, and protists.

* Prokaryotic cells, on the other hand, are simpler cells that lack a nucleus and other membrane-bound organelles. Their DNA is located in a region called the nucleoid, which is not enclosed within a membrane.

So, to summarize, all eukaryotic cells have a nucleus, while prokaryotic cells do not.