Because he has Asperger syndrome.
You don't know what you're talking about. Abed isn't portrayed as being simply 'socially awkward'. He doesn't pick up on social cues; common sense things like that people should take offense to having their flaws crassly exposed are strange revelations to him. He has a narrow band of interests which he pursues obsessively. He has trouble grasping nonliteral usage of language, and his own speech is verbose, and generally lacks the intonation and flow people use to superimpose further meaning over their words. In brief, he couldn't be more obviously characterized as having Asperger syndrome.
There's a growing sentiment among people, mostly people who don't know what they're talking about, that people diagnosed with an ASD are "just socially awkward". Some of them are. A lot of them aren't. Thing is, this sentiment coincides with an intolerable trend whereby social awkwardness is construed as a desirable trait; it's quirky, and funny, and hip. For people with an actual ASD, it's not really any of those things. It's awful. It prevents people from developing human connections in the same way other people do. It's not an affectation used to get laid, the way so many people have come to think thanks to indie movies.
In my opinion, Abed is one of the most sympathetic and insightful portrayals in popular media of someone with a genuine ASD. But he's also a very sad character. These aren't competing realities; they're complementary ones. Granted, in some instances Abed has broken this mold. He was shown to be popular with women, and fairly adroit at fielding them. He doesn't have trouble approaching people, or being in many social situations. This isn't absolutely inconsistent with Asperger's, since obliviousness can be emboldening, and it can be seen as a best case scenario for someone with the disorder. The ways in which Abed is different are a source of humor, and they aren't often dwelt on, but they very much define his character, and they are consistently handled with a good deal of unpatronizing understanding.
I don't mean to sound like altogether too much of a dick, but if you're speaking from experience, you may well know how it feels to go to a party and feel uncomfortable and not have anyone to talk to. You likely don't know how it feels to absolutely and without exaggeration never even go to a party, because the idea explodes in your head and that's that. You probably don't know what it's like to have just a single friend, or no friends at all, and to not even be able do things with them. Michael Cera in Arrested Development, or Michael Cera in Superbad, or Michael Cera in Juno, these are characters which are "just socially awkward". Abed isn't "just socially awkward". Abed has Asperger syndrome. It's not wacky. Trust me.
Educate yo'selfs! 
