Here's why:
* Momentum is a scientific term, not a common noun like "dog" or "chair."
* Scientific terms generally don't have foreign plural forms. They are usually used in their singular form, even when referring to multiple instances.
If you need to refer to multiple instances of momentum, you can use phrases like:
* "several momentums"
* "different momentums"
* "multiple momentums"
However, in most scientific contexts, you would simply use "momentum" even when referring to multiple instances.