The Talmud teaches that Avraham kept the entire Torah before it was given to the Jewish People at Sinai.
Medrash says that Yitzchok kept the laws of shchitah (kosher slaughtering), and Yaakov the laws of Shabbos - before the giving of Torah at Sinai. Truthfully, Yitzchok and Yaakov kept all of Torah, just as Avraham did. Why, then, is only Avraham mentioned as having kept all 613 mitzvos? Torah's goal is to create a world of chesed: a world of giving and of kindness.
Avraham's defining character trait was the same: loving-kindness (chesed). Things being so, Avraham had an innate connection to Torah. Avraham, by his nature, was closer - more similar - to the laws of Torah than were the other Patriarchs. Avraham, specifically, was bound to Torah, in a way that left him alike one commanded to keep all of Torah.
Hashem appreciates more a mitzvah accomplished by a person commanded to fulfill that mitzvah, than when accomplished by someone not so instructed.
[The reason Hashem instructs the person that He does, is because He more desires that person's doing it!]
Thus: Avraham's observance of Torah was of more significance than its being kept by Yitzchok or Yaakov. And thus: Avraham, specifically, is mentioned as having kept all of Torah!
(Maharal: Tiferes Yisroel; Chapter 20)
