The United States's No. 4 semipostal stamp: Save Vanishing Species (Scott No. B4)  was originally sold nationawide
from September 2011 through December 31, 2013. The surtax was donated for multiple national species conservation
programs.

But only 25% of stamps were sold (25 million out of 100 million stamps printed)

The USPS then decided to place the stamps back to sale until December 31, 2018.

adapted from Linn's Stamp News
Published 10/13/2014

semipostal stamps are originally designated for fund raising.
I just wondered why the USPS set a deadline for the sale