Because at the time of Christ, there was an 'avuncular society', which protected women.
The brother of the deceased was expected to provide sustinance for the widowed wife and children; there was no implied or conjugal relationship(implies sexual relationship).
But that wasn't the case in Joseph Smith's case. Many of the women he married became pregnant with his child.
And, Jesus never said that polygamy was required to enter into heaven. It was Brigham Young and Joseph Smith that did.
That doesn't explain Smith marrying so many young girls who never married nor does that take into account the children he had with them.