They are worried there is no way to know how many guns are in circulation.
Check it out:

Empire state politicians gauge gun registration deadlines come and gone and enforcement of Gov. Andrew M. Cuomo’s landmark gun control law they say is unconstitutional and may cost him re-election.

“New Yorkers are blind to how many people registered their ‘assault weapons’ by the April 15 deadline,” said William R. “Bill” Nojay (R.-Pittsford) state assemblyman representing parts of Monroe and Steuben County and all of Livingston County in Western, New York.