Man arrested for 2x4 labeled "High Powered Rifle"
Because the majority who support universal background checks are silent and generally don't vote when it comes time to put new NRA leadership in place (I believe the percentage of NRA members who actually bother to vote for their leadership is around 3%). The more extreme someone's views are, the more likely they are to cause a stir within the organization and to cast a vote. The squeaky wheel gets the oil, and within the NRA, the "no regulations whatsoever" wheels are very squeaky and the moderates tend to be pretty quiet. This happens with a lot of political parties and activist groups; it's not a dynamic that's unique to the NRA.
Remember: we already have universal background checks for NEW firearms, just not USED.