It's much easier to predict results if you're supply constrained.

Demand is an emergent, fickle phenomena: the swarming logic of how people in the wild actually behave.

One reason it's fickle and hard to predict is what people say they will do is often different than what they actually do.

"Oh yeah of course I'd find a product with those features compelling and be interested in buying it" but then it comes down to time to buy it they say "Oh we don't have the capacity right now to consider other suppliers, even if they are theoretically better."

The ultimate test of demand is "do people actually buy the product at the real price."

Supply is much easier to predict based on the costs and capacity of your input suppliers.

This is more intrinsic and reductive.

You can take for granted that if a supplier says they can make X widgets at Y price you will most likely get it.

This means that supply sets a kind of foundation and demand sets the emergent ceiling.

It's easier to test supply and get handshake deals to lock in a steady foundation than it is to test demand.