The command to “search the prophecies of Isaiah” is one that is repeated often in the Book of Mormon. Moroni merely commends them, noting that “I cannot write them.” The inability to write them had to be related to the room available on the plates, not Moroni’s abilities. Moroni also knew that much of Isaiah was on the small plates that he knew would be appended to his father’s record. Therefore, there was no need to write them.
Why does Moroni commend Isaiah? Isaiah prophesied of a gathering, and the Book of Mormon had been foretold as one of the precursors to that final gathering. God made a covenant, and God keeps his covenant.