Without going down the semantics rabbit hole, I'd say that it is more rare to find good #1 defense, and that teams don't part with it when they have it.
When you don't have it, you know it, and you covet it. You also find that acquiring it in trade is exceedingly difficult.
Drafting it is not assured, when you're at the nadir of your competitiveess curve, and even when you do, it takes many years to develop a #1 defenseman.
To me, centers are more plentiful, by contrast, and they reach their prime earlier.
Now there is another wrinkle to this. Increasingly, when we debate the makeup of Cup contenders, we like to see teams that are in the elite category with two #1 centers, a 1a/1b sort of scenario. Few contenders ever reach two #1 defensemen. Anaheim in 2007 and San Jose this year are the only two teams that I can recall in the cap era. That may impact the calculus. Don't know.