Why aren't Bilhah and Zilpah considered matriarchs? If Yaakov married them, obviously they were great people, right?
Rachel intended for Bilhah to be something of a "surrogate mother" - the children that Bilhah would bear would of course be biologically hers, but they would also be considered Rachel's. [She thus asks Yaakov to marry Bilhah, so that "she will give birth upon my knees" - as Onkelos explains, "I will raise the children" - "and I will thereby be built up through her" (Gen. 30:3).] Leah did the same with Zilpah.
So generally speaking, Bilhah's and Zilpah's contributions to the Jewish People are subsumed under the names of their respective (former) mistresses, Rachel and Leah.
That said, Arizal (Shaar Hapesukim to Deut. 4:20, et al) states that in several places where the Torah mentions ברזל, iron, it hints at the four mothers בלהה, רחל, זלפה, לאה, whose initials (in this order) form this word. So we see that they do indeed rank as mothers of the Jewish People (and indeed, note that they precede Rachel and Leah in this enumeration).
A parent isn't only biological.That is why you can refer to the parent who raised you as your father on a Kesuva with the qualifier "HaMegadel".
The four mothers participated in the Holy Education of their children, including the other four children of Bilha and Zilpa. Hence, although Bilha and Zilpa were great women, their role in the education and Holy development of their own children was secondary to Rachel and Leah's influence. This is why we all consider Rachel as our Mother, even though most of us are children of Leah.