Normal people don't think "how many CPU cycles does this DE use?". They look for familiar, frictionless, easy to use and pleasant to the eye interfaces. If you're happy with dwm, good for you. I'm happy with xmonad. DE are made for the majority of people, not for nerd.
Also "lightweight" gets too much play in Linux UI circles. What people actually mean, I think, is "not janky and broken".
Heck, why is Linux Mint exciting? Because it's explicitely not trying to revolutionize anything, they're just trying to make the basic idea work as well as possible.
