We didn't want this for three reasons: we'd lose out on the marketplace cut (10% of all sales I think?); we didn't want people grinding the game to earn money from rare drops; and finally because 3rd party trading ended up creating a lot of scams and therefore angry players.
At the time, we didn't see any way around it: we couldn't prevent people "gifting" items to each other, and despite omniscience and omnipotence in the game and Marketplace, we weren't confident that we could rejigger the drop rates and rarities to lower the maximum perceived value of the fanciest knife to be under the $300 limit.
I suspect that the CS:GO team finally decided to do something about it and chose this. If the team is anything like I left it, they probably modeled this extensively (we had data on nearly every game ever played in CS:GO and complete Marketplace data), and discussed the change with the TF2 and DOTA teams, who also have to deal with this, and decided that the short-term fury of a small fraction of the playerbase was worth it. I wonder if TF2 and DOTA are having similar problems and, if so, whether this change will be rolled out for those games, too.