Because ME2 had new romances and it worked out well.
Because there will be people who will be introduced to the series through ME3.
Because a lot of old romance options aren't going to be on the Normandy.
Because Bioware realized that they were being kind of ****ish about s/s romances, particularly of the m/m variety, so they decided to fix that without upsetting anyone ("character integrity, retcon, blah, blah..." (protip; s/s romances will still upset most of those people anyway)).
Because people like new things.
I personally don't have a problem with new characters, even if they are there just so that Shepard can sleep with them, as long as they at least appear to have some other role in the overall story and war.