Maybe because back then it with crappy internet it would be hard to download any downloadable content (dlc) so it was easier to sell it in a disc as an expansion for the original game and thus selling more?
I am glad we still get additional content for games, just not that nickel and dime crap.
Expansion packs still exist. DLC is just bigger. A lot of smaller overpriced content or 1 big piece of content that is reasonably priced. Companies want money and people are stupid and will buy horse armor for 5 bucks.