The reason why most projects are campaigns is pretty simple: Because they're easier to devise, challenging to make, and they last longer.
Let me elaborate: What is more fun in making a SCII project? Devising an universe and it's characters, work these to be identifable and easy to relate with, and put up a simple, straight adventure which ultimately alters their status quo. It can be a spoof/comedy like 'Bob the Cerebrate' or something epic like 'Flame Knives', but it's no less engaging to see fresh new faces, concepts, ideas and/or places other than those we're already so used to and tired of. And all those notes produced in Notepad, MS Word and others somewhat do feel like some progress was achieved, because we think about them, we reason, we balance the inifinite number of variables related to about everything.
But it's no less challenging to make, and that often depends on the person's personal aims: How far is he willing to go, to make this story? How much can he do and what does he want? And above all, the ultimate question: Will I ever be able to finish it? This is quite common, especially in StarCraft campaigns, just look at all the EDAST series and you'll see what I mean. Often it's just too much work, or there are limitations, or even the worst-case scenario, constant, unrewarding frustration. That's why devising is fun, but often must be done VERY carefully, considering not only the potential, but especially limitations.
Finally, the reason why they last longer, besides story, mechanics or characters is because they often offer a much more stronger experience than Arcade maps do. Arcade maps, true, are much more quicker and easier to make, but it's also a part of problem, because they're just minigames, they're just something for you to waste some time, and unless they happen to be very good or very interesting, they don't last long with audiences and aren't as inspiring as the idea of 'telling a story' or introducing something relatively new. Campaigns, on the other hand, have a greater ton of work, but feel ultimately rewarding for both the audience and the developer himself when it's finished.