Because popcorn is an uncountable noun.

Word analysis:

popcorn Pronunciation: British ['p?pk?n] American ['pɑ?pk?rn]

n. Popcorn? adj. (Things without artistic content or wisdom) Popular

Word collocations:

1. popcorn noise jumping noise

2. popcorn machine popcorn machine

Bilingual examples:

I bought a cup of coke and some popcorn.

I bought a cup of Coke and some popcorn.

Extended information:

Synonyms

corn　

Pronunciation: British [k?n] American [k?rn]

Corn is an uncountable noun. In British English, it means "cereal; grain", especially "wheat". In American English, corn means "maize, corn".

Corn can also mean "mediocre, sad or corny music, poetry, drama, etc."

Vocabulary collocations:

1. an ear of corn

2. corn cake

3. corn bread Corn bread

Bilingual examples:

The ground was plowed and planted with corn.

The ground was plowed and planted with corn.