The legislature turned this down because it was the general consensus it would be unfair to single out polygamists...
You may be right that the authorities are reluctant to prosecute polygamy involving consenting adults. Politicians still remember what happened to the governor of Arizona back in the 1950s:
Gov. Howard Pyle ordered a 1953 raid on the Colorado City area, then known as Short Creek, that resulted in the arrest of polygamist husbands. Most of the criminal indictments were eventually thrown out, and bad publicity from the raid was blamed for ending Pyle's political career. (Arizona Republic, 2 October 2002)
(Colorado City, Arizona, is just over the state line from Hildale, Utah. Both towns are populated by polygamists.)
However, the state of Utah is cracking down on polygamy involving underaged girls. The legislature recently passed a law that makes "child bigamy"—cohabitation or marriage between a married adult and a minor — a felony. There have been arrests and proescutions under the new law.