One obvious answer is that sed is simpler, i.e. easier to learn.

So for people who don't do a ton of cli or text manipulation, it might make better sense to learn sed in a weekend rather than devote more time to awk.

I should say that I don't have very strong feelings for either one. I tend to use sed for simple things, awk for slightly more complex things, and then Perl or Ruby for proper things. But I think your all-or-nothing approach here is a bit ham-fisted. Nearly every *nix system will have both. There's no need to choose between them.

easier in what sense? lines of code? regex is easier to learn?

yes nearly every *nix have them, but you only need to use awk. this will cut short one's learning curve.