Here's why:
* You can't count it: You can't say "one toothpaste, two toothpastes," etc.
* It's a substance: Toothpaste is a substance, like water, sugar, or air.
* We use "a tube of" or "a bit of": We use quantifiers like "a tube of" or "a bit of" to describe amounts of toothpaste.
Examples:
* I need to buy some toothpaste. (Correct)
* I need to buy two toothpastes. (Incorrect)
* I used a tube of toothpaste this morning. (Correct)