Seems pretty simple to me. When there's less demand for something than you thought, you put it on sale.

They've sunk resources into the S houses. They want to see them in use. They need to entice people to buy those, and cutting the prices is the simplest thing they can try. If price cuts don't get them selling enough, they'll likely try other enticements, but price cuts are an easy first step.

They don't need to lower M and L prices, because they know there's enough demand for those to begin with. FCs are going to buy them, even if they haven't yet.