Portal is one of those games that just kind of came out of nowhere. It was included in the Orange Box and just seemed like an extra bonus game. However, once you started playing it, you realized that the game was actually compiled of pure awesome. There were only two issues that I had with the game, one was that it ended, and the other was that you could not purchase it without getting the entire Orange Box. Valve corrected this a short time later for PC gamers, however, Xbox 360 owners aren't going to be as lucky.
Apparently Valve has had every intention of releasing the game via Xbox Live, but unfortunately it was rejected by Microsoft. It's not that they didn't like the game, but there were technical factors that prevented it from being offered.
As Doug Lombardi at Valve put it “I'd love to sell Portal on Xbox live, [but] the platform holders aren't doing that right now. There's a size limit and all kinds of other things.” Hopefully something will be figured out in the future that will allow this masterpiece to become available. After all, Microsoft did up the size limit from 50MB to 150MB, maybe they'll up it again.