Because it takes an amendment to do such a change and it would have been a political outcry from Republicans. Obama would probably had gotten lynched by even making the suggestion. Obama was also too much of a moderate to make such a change. That's why we needed someone more radical as Bernie to make such changes. Maybe this time around if Biden wins, and we also win a great number of seats in the Senate and also the House then Democrats stop playing nice and push for such a change but it will take 2/3 of them to approve such a change so it is not an easy task.