Why can't I make Polders on Wheat floodplains?

Title says it all.

Because wheat can only be improved by a farm, and you can't build anything on a tile with a bonus resource (like wheat) other than whatever improves that resource (or a fort -- you can build a fort anywhere). You also can't build a polder in a floodplains tile with incense or a marsh tile with sugar.

I think the moai is also an exception, but, yeah, exactly as Browd says, the vast vast majority of resources, bonus or lux, can only be improved with the "proper" improvement, forts, moai, and great people improvements are the only exceptions.

The pain when you see lots of bananas as Brazil and just dream of being able to put brazilwood camps on them (or anyone else, trading posts would be fine).