Because he's presenting this clarification that the gospel is open to everybody, it is no longer an exclusive thing for the Jews, that it's a new covenant, we're not under the old covenant anymore. That he starts off, it may just sound like an innocent thing, which he had promised beforehand by his prophets and the Holy Scriptures. When he says Holy Scriptures, when he's writing to the Romans, the Holy Scriptures meant the Old Testament. There wasn't a New Testament yet.
It was being written, some of the letters were circulating, it was beginning to form, but when Christians thought of the Holy Scriptures, they thought of the Old Testament. So he's saying all of this was promised beforehand.