Here's why:

* Flagella are used for movement. Cocci are generally non-motile, meaning they don't move on their own.

* Other bacterial morphologies (like bacilli, which are rod-shaped, and spirilla, which are spiral-shaped) can have flagella and are often motile.

It's important to remember that there are exceptions:

* Some cocci can be motile and have flagella, although this is less common.

* Some bacteria don't have flagella but can still move using mechanisms like gliding motility or twitching motility.

So, while cocci usually don't have flagella, it's not a hard and fast rule.