Onion vs onions

Today I came across a sentence in The Daily Star

prices of locally grown onion rose yesterday for the lack of availability.

I know onion is countable.Therefore,it should have been locally grown onions instead of onion.

Why onion was used rather than onions? Can anyone please help me get a clue

As a native speaker, if I were speaking I would have said 'onions' as you expect. And I think that would be the usual way to say it in formal register, 'the price of onions'.

But using 'the price of onion' isn't wrong. It just makes it sound like they're referring to the general concept of the crop or substance. Think of it as a noun modifier 'the onion price' (as the Saxon genitive) and converting that to the Latin one, 'the price of onion'.