User blog comment:Aleksandr the Great/Prothean Squadmate is Officially DLC/@comment-1221773-20120222232942/@comment-1221773-20120223002251

Key words there are he seems way too important. Maybe the issue isn't that BioWare made the Prothean into a DLC character, but that they wrote ME3 in such a way that the role of a living Prothean was deemed unimportant enough to warrant being included in optional DLC. If the Prothean had been given a bigger role than whatever he has by the writers, doubtless he would be in the main game.