This question is asked by the Toras Hamincha (a student of the Rashba) in his drashos (Lech L'cha drasha 8). Another student of the Rashba R. Yehoshua ibn Shu'aib writes that many have asked this question (parshas vayeira and drasha to Parshas Tzav / Shabbs Hagadol). He writes that some answered that Avraham refrained from doing so because the merit for a mitzva is greater if one is commanded in it and he knew that God would command him in the future so he held off until he was commanded. He quotes others who explain that without a divine command he wouldn't have circumcised himself, for by doing so was tampering with the form that God had granted him which would be an insult to God.