The Mechaber, Rabbi Yosef Caro, in O"Ch chapter 168, rules that bread will be converted to Mezonot if it is altered in any one of three ways. Either the bread dough encapsulates something like fruit, nuts, meat etc; or the dough has a sweetener added to it, like fruit juice etc; or it is baked into a hard wafer.
This last option is that of Rav Hai Gaon; plain bread dough, when baked as dry crackers which is known as "Biskonis", are Pat HaBaAh BeKisnin; what is today colloquially called "Mezonot".
The Mishneh Berurah (bio), 35, emphasises the point that this is a plain bread dough that contains nothing other than flour and water. It is demoted from being HaMotzi simply by its final baked form being a hard, dry product. This is exactly what our Matza is.