Well, I've never seen a track fork with disc tabs....I think you'd have to run either a cyclocross fork or some sort of clamp adapter...or maybe something really cool, that I don't even know about.

As for why/why not...there's not really any reason to. Not any reason not to either, besides overkill. I run discs on both my mountain bikes, but have zero use for them on my fixies, rim/no brakes are just fine.