The Jews did not understand the law because they were missing an essential piece, a true understanding of the atoning mission of the Messiah. The hardness of their hearts against this atoning mission is something that would not have been included in the brass plates. While the brass plate texts certainly contained prophets calling Israel to repentance, it is never clearly because they rejected an atoning Messiah. Indeed, for much of its history Israel expected a conquering Messiah rather than an atoning one. Abinadi's evidence for the hardness of the hearts of the Jews concerning this part of the gospel is interesting as it can come from only two discernible sources. One would be direct revelation. The other would be the writings of Nephi. On the small plates that we have, Nephi records information specifically related to the Jews rejecting the atoning Savior. Both his father Lehi speaks of this (see 1 Nephi 10:11) as well as Nephi himself (1 Nephi 15:17).
While it would be tempting to say that Abinadi had access to a copy of the small plates, this is very unlikely, as the plates were probably given to Mosiah after the departure of Zeniff (Noah's father) or the land of Lehi-Nephi. However, if Nephi recorded his father's words in short form in 1 Nephi 10, it is quite likely that they existed in fuller report in the Book of Lehi (on the large plates). It is possible that a copy of this work as well as the brass plates accompanied Zeniff and was a primary text for his priests (again speculating that Abinadi was one of those priests and therefore had access to this information.
It is interesting that Abinadi refers to this stiffneckedness in the past tense, when the evidence for it lies yet in the future. Since the prophecy is in the past, and for Abinadi the word of the Lord is sure, this future event is as good as accomplished.
Rhetorical: Abinadi is pounding home the essential point of his discourse, that the mission of the Messiah is essential even for the law of Moses. He is continually underlining that the religion of the priests cannot be effective to save precisely because it denies the Savior, as did (will!) the Jews.