What's different now? This time HaShem appeared to Avraham it is after the Brit Milah. This, the Ohr HaHayyim explains, is the answer to our question, as expressed in Midrash Rabbah, the Avot, the forefathers were the Merkavah ("vehicle") for the Divine Pressence in this world. God revealed Himself upon Avraham Avinu.
Even more than that, all the while that Avraham bore the Brit Milah, HaShem never left Avraham Avinu and so this is the last time that God is recorded as 'revealing Himself' to Avraham before speaking. From now on, God simply speaks to Avraham without needing to be 'revealed' again. The revelation in the beginning of Parashath VaYeira is a lasting revelation.
[Finally, he quotes the Zohar which says: as long as the mark of God (Brit Milah) is upon a person, the Divine Presence rests upon him.]