Brochas Daf Tes Zayin amud beis

The Rabbis taught that we do not call anyone Avos except for three (Avraham, Yitzhak and Yaakov) and we do not call anyone matriarchs except for four (Sara, Rivka Rachel and Leah)

The Ben Yehoyda adduces proof for this statement from two sources, one textual, one kabbalistic.  The textual source is that the oldest of the shevotim (the tribes), which represents the generation after Yaakov is Reuven, which he darshans as Reuh Ben, "see, a son" - in other words, not an Av, a patriarch.  The kabbalistic rationale is based on the observation that the Shem Havaya, (Yud, K, Vav K) has the numeric value of 26, as does the number of letters in the combined names of the patriarchs and matriarchs.