Orson Hyde says plural marriage was kept secret to protect the Saints practicing it.
Orson Hyde, "The Marriage Relations," Journal of Discourses, 26 vols. (Liverpool: F. D. Richards, 1855), 2:83
"Well, but," says one, "there was certainly an injunction laid upon the Bishops in New Testament times, that they should have but one wife. This is brought up as a great argument against the position the Latter-day Saints have taken. In olden times they might have passed through the same circumstances as some of the Latter-day Saints had to in Illinois. What would it have done for us, if they had known that many of us had more than one wife when we lived in Illinois? They would have broken us up, doubtless, worse than they did. They may break us up, and rout us from place to another, but by and bye we shall come to a point where we shall have all the women, and they will have none. You may think I am joking about this, but I can bring you the truth of God to demonstrate it to you I have not advanced anything I have not got an abundance of backing for. There is more truth than poetry in this as sure as you live.