If you removed all barriers on agricultural products being sold to developing countries, their local agriculture would be pancaked by cheap wheat, corn, and rice grown in more agriculturally productive regions.
- access to capital. A modern family farm has several million dollars worth of land and several million dollars worth of machinery. Farmers don't buy $500,000 combine harvesters for no reason -- in the long run they're cheaper than the alternatives.
- security. A modern farmer just leaves millions of dollars worth of machinery in his yard, usually unlocked.
- information. In developed chemicals the government & universities put out lots of information about techniques, seed varieties, chemicals, machinery, timing, weather forecasting, et cetera
I can't speak to the state of information and knowledge of agriculture in the various countries we'd consider "developing," and how it compares to universities and government here, but I bet it varies quite widely. That's a good point, though - it matters a lot.
But as you sort of alluded to, in a free market some areas could put downward price pressure on corn even if they're not as efficient as Iowa. If they've got land that isn't very well suited for anything else, and it's a choice between making a little money on corn and little or no money at all with their arable land, they'll probably go for it...