Gods had exarch that fulfilled the more precise or niche concept, much like Nusemnee who was the exarch of forgiveness and redemption. The great gods, as presented, are more the gods of high concepts.
And, like mentioned, many gods were slain during the war and the portfolio are not that easily assimilated when compared to FR gods who gain and lose portfolios each week.
And since the gods are barred from acting directly in the setting, its pretty hard for any new deity to gain traction enough to gain enough power to take more domains. I think some Exarchs and the Raven Queen were the more recent examples.
As for agriculture, like in most polytheistic setting, it makes more sense to split the concept over to many gods:
Avandra during spring for a quick thawing of the earth, Pelor in summer for good crops, Sehanine during the harvest. Mellora would need some offering to keep her beasts and tempest away from the field for the season. A small donation to the priest of Tiamat to keep in check the merchant's greed when you buy your seeds or sell your production could be required, or to the themple of Zehir to avoid spoiled harvests etc