referring to how SC1 has hundreds of awesome customs games which had me coming back for years, and then SC2 which had me until I basically finished the campaign. Also can be said for CS:GO. The custom games in Source were amazing an ingenious sometimes.
The sc2 editor is rather complex (which is a good thing) but you need a lot of skill and time to master it totally. Many people who has good map ideas don't neceserally have the time nor the skill to create something decent with it, and those who has the skills may not have good ideas... plus the time you need to create something amazing is not often rewarded
But Blizzard could easily improve that by allowing more premium maps and do a bit of advertising on the BNet app
If you ask me, it's because they never took off in the first place. As most people know, SC2's custom games scene, early on, was dismal. The interface for finding hosted games made it so getting a map recognized was exceptionally difficult. It got much better, but I believe it came too late.
The relatively high skill floor of using the editor didn't help. Blizzard didn't start to show real progress in trying to improve the editor for newcomers until the last major patch, which was never fleshed out. The game was shortly thereafter announced to be entering maintenance mode.