That one is caused by zoning.
If I can make a low-rise building and sell 12 units for $250k on the same property that I can build two detached houses to sell for a $million, I'll do the former, right?
But city hall is going to make me drag out the approval process for the low-rise for 3 years and grind me down to 6 units. I'll just save the ball-ache and build the mcmansions.