Technically it's the easiest object to apply a texture mapping. Most of the cloaks are rectangular or trapezoidal, with some bending here and there, but the core object remains a low-polygon rectangle type. Since the form is so simple applying a texture is quite easy and doesn't need much preproduction work so when it's applied to the object it looks correct (you need to stretch the texture to adapt it to the form of the object. Rounder objects like heads need more work so when the skin texture is applied it doesn't look kinky).
So we start playing our level one character, cloakless. By killing the first mobs we'll get some grey-quality items to equip and one of them will be a "cloak". Sorry but that barely can be called a cloak. Maybe a neckerchief or napkin tied on your back.
As your character progresses you'll start getting longer cloaks, capes or mantles. First ones, white-quality and still without any stat are still too short to be called cloaks. Again it seems you have a handtowel tied on your back. The first greens you'll get will also be still short and simple in design.
As time passes your character will get longer cloaks but unfortunately not better in design. Vanilla WoW have simple designs, almost patternless. Some cases are really painful, like getting a blue quality cloak named Tigerstrike Mantle... that even doesn't has tiger stripes on it so you can't avoid feeling deceived when you get it.
During TBC things didn't get much better and only in endgame we can see some elaborated patterns like the Shroud of the Highborne, but the design, as the major part of TBC gear is somewhat... too colorful. I wonder sometimes if TBC designers were thinking on the Acid House times. The only cloaks orth mention are the blue rewards from the Death Knight's questline. And even these don't make cloaks look like they're made of cloth.
And then we reach the Wrath of the Lich King expansion were things haven't improved much. First endgame cloaks from Naxxramas are hideous. Aged Winter Cloak? Horrible colour combination. Cloak of Armed Strife? Come on! Not only has again a more than arguably colour choice but that "skull" motiff on it like Superman's cloak it's just a big design fail. And after all the patches few cloaks are worth mentioning, like Aetha's Intensity. That's a cool looking cloak! Maybe too ellaborate, but at least is worth displaying it in your character.
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So here's my calls to Blizzard designers: please improve cloak designs. Forms are ok (you can add more complexity like how it bends and moves), but patterns need more work. And you don't really need to add complex designs like the Drape of the Violet Tower. Check animal hides and create a good tigerskin cloak, not that Tigerstrike Mantle.