You ignorant son of ...
j/k
Long version (short version -> scroll all the way down):
Well, it's just that i like it, and compared to some other mods, like pure sound mods or BMS (even though it's still hard work) they made something new. Not in 2008, though, thats true.
But what did most other mods do? BMS released a trailer. Is that enough for mod of the year? Don't think so. Once they release BMS itself, who knows.
I just skipped through the "released" mods list of moddb, ordered by release date. Of the 10 first pages (starting with newest release) i only ever heard about Obsidian Conflict, and that had some serious issues.
Or *"TheCheese33"'s* Left 4 Dead - Randy Savage Infected Sound Mod. Released 30th December, never heard of it before, only sound mod from what i've read. Even a whole game like L4D itself doesn't make that huge impact in 2 days to warrant a "game of the year" award (as it had more, who knows, but if it only had 2 days..).
I admit i'm not going through new released, may it be mod, trailer or whatvever each day, but if the average "mod-interested" hasn't heard about it, it's unlikely that it was that fantastic.
Also i think we all know everybody is going to vote for his favourite mod anyways, and not for the one who accomplished most. It never works like this with such awards, may it be mod, game, movie or whatever of the year.
Short version:
Dystopia did accomplish about the same as most mods. If a mod isn't well-known, it's either not that good, or if released too late -> vote for it 2009, once it showed whats behind, after the initial "woah, something new" is gone. Also votes will not show who accomplished something fantastic, anyways. Black Mesa would not be the current #1 of the HL2 Top 100, as it's not even released.
*edit*
Also, you are doing about what i said everyone would do. You are voting for a mappack (it's not even a real mod) and Sven Coop Beta. Thats not even a full release. So lets take a look.. Dystopia has currently a closed beta for 1.2, (and they are only waiting for Valve for Steamworks), so there is what you missed, they did something fantastic!
Not that i wouldn't like coop etc, it's just that in my opinion it proves what i said about what people are voting for.

Half Life: Decay is also good mod.

Yeah, that's retarded that they just open up any and all listings of their database to be counted.
As good as BMS looks, a trailer isn't enough to be counted the mod of 2k8.
I mean, does that trailer provide the same kind of fun and action you get from playing an actual mod as opposed to just watching someone else play it?
No, I don't think so.

Agreed, BMS should definitely not win MOTY2008. 2009, however...
You gotta admit, if it is released in 2009 and if it does live up to that trailer...
Whoa...

Yes, there would have to be an immense amount of competition in order for it not to.
I still have some hopes for some other projects though.

I don't know... maybe the problem was that I played a lot of Quake before. I got lost a couple of times, and plenty of times I couldn't see the guy who was shooting at me... I really like my shooters rather primitive, because that's what I grew up with. Dark corners for enemies to hide in are a big no-no. I don't really seek a challenge when playing a shooter. I seek atmosphere.
I see the path, I follow the path, I see enemy on path, I shoot, he dies. Sometimes I don't see him because he's hiding, but when I do, he'll be right in front of me.