Why not many greater quantities or a much greater quantity?
More land is being diverted from local food production to “cash crops” for export and exchange; fewer types of crops are raised, and each crop is raised in much greater quantities than before.
"Much" here is an adverb functioning as an intensifier, meaning "considerably", "to a large degree or extent". You can pretty much replace it with "a lot" without changing the meaning of the sentence. "Considerably" and "significantly" are also good substitutes.
She is much older than him.
= She is a lot older than him.
= She is considerably older than him.
= She is older than him by a lot.
"Quantity" is a count noun here meaning "number".
People came in great quantities.
= People came in great numbers.
= A lot of people came.
"In quantity" can also be idiomatically used there, and there is no real semantic difference between "in great quantity" and "in great quantities".
What Eddie Kal's answer doesn't include, is that "much" is an intensifier of the word "greater", and not of "quantity" — which might have caused your confusion.
Idiomatically, one could even say that there are 'many much greater quantities', when, in comparison to another situation, the 'quantities' are both 'many' as well as 'much greater' (than in that other situation).
The use of this particular phrase (combining both 'many' and 'much' — but strictly in that order) would be very circumstantial, and probably initially strike most readers as remarkable, but it could definitely be used correctly.
