I'm a right-handed person who just picked up the guitar a few months ago, so take this as you will.
I think sort of the idea behind it is that by using your dominant hand for the the more simple work, it allows you to focus on the fretting. You can naturally match fretting with your dominant hand on strumming pretty easily, so you barely have to think about it. If it's the other way around, even if fretting by itself is at all easier, doing it in combination with the strumming is now harder, because you have to use your non-dominant hand for strumming, and thus have to think about it more. It becomes harder to do both tasks in unison.
Also, I find that strumming takes more physical strength than fretting.
I dunno, that's just my take on it.