"for some reason"
The reason is that "universal background checks" = "universal gun registration".
Here are 13 reasons: LINK
FIRST: The principle that no American can own a firearm without getting the go-ahead from the government is offensive to Americans. ...
SECOND: Universal background checks would not have stopped Adam Lanza (who stole his guns), or James Holmes or Jared Loughner (who passed background checks).
THIRD: One of the nation’s leading anti-gun medical publications, the Journal of the American Medical Association (JAMA), found that the Brady law has failed to reduce murder rates. ...
FOURTH: Increasingly, the Bureau of Alcohol, Tobacco & Firearms is illegally going into gun dealers and xeroxing all of the 4473’s -– a practice which represents the beginnings of a national gun registry.
FIFTH: The FBI refuses to tell us how or whether it is complying with the Smith and Tiarht amendments prohibiting Brady names from being kept for a national gun registry.
SIXTH: Germany just set up a national gun registry, based on index cards kept at 551 locations throughout the country –- cards not dissimilar to the 4473’s which every American gun buyer would be required to fill out under a Universal Background Check system.
SEVENTH: The NICS list currently contains the names of more than 150,000 law-abiding veterans who didn’t do anything wrong (but honorably served their country and then sought counseling for their wartime experiences) ...
EIGHTH: Anyone who doesn’t believe a national gun registry leads to national gun confiscation should consider the confidential memorandum advocating confiscation and circulated by New York Democrats prior to their most recent round of New York gun control.
Whether they know it or not, anyone who supports "universal background checks" supports gun registration. It is impossible to have an enforcable "universal background check" system without universal gun registration.
There is a resaon why Schumer, Fienstein, McCain want registration, and there is a reason why gun confiscation plans are preceded by registration.