Because 10% of a Ridiculously Huge Stockpile is still
a Huge Stockpile
Although April 2012 is the latest date allowed by the treaty, the U.S. also noted that this deadline may not be met due to environmental challenges and the U.S. decision to destroy leaking individual chemical shells before bulk storage chemical weapons.
The primary remaining chemical weapon storage facilities in the U.S. are Pueblo Chemical Depot in Colorado and Blue Grass Army Depot in Kentucky. These two facilities hold 10.25% of the U.S. 1997 declared stockpile
According to the U.S. Army Chemical Materials Agency by January, 2012, the United States had destroyed 89.75% of the original stockpile of nearly 31,100 metric tons (30,609 long tons) of nerve and mustard agents declared in 1997.
The Pueblo Chemical Depot is a chemical weapons storage site located in Pueblo County, Colorado, United States.
The depot houses 2,611 tons (2,369 metric tons) of mustard agent in approximately 780,000 munitions, equivalent to about seven percent of the original chemical material stockpile of the United States. The current course of action for the Pueblo Chemical Agent-Destruction Pilot Plant, included in the President's Fiscal Year 2010 Budget Request, will allow for acceleration of the project schedule. After systemization and pilot testing of the facility, full-scale destruction operations are expected to begin in 2014 and to be complete by 2017. The plant will operate until all the chemical weapons have been destroyed. Closure activities (shut-down, dismantling, and restoration of site) are slated to be wrapped up by 2020. The plant will use neutralization with a hot caustic solution followed by bacterial biotreatment to destroy mustard agent.  This is far outside
the deadline set by the Chemical Weapons Convention (April 2012).
https://en.wikipedia.org/wiki/Blue_G...on_Pilot_PlantBGAD stores a small stockpile of chemical agents, comprising 523 tons of nerve agents GB (sarin) and VX, and mustard gas, or about two percent of the United States chemical weapons stockpile.
Beginning in 2003, destruction of the Blue Grass chemical weapons stockpile, in accordance with the Chemical Weapons Treaty, was contracted out using a technology known as neutralization followed by supercritical water oxidation. This is a different method than the incineration that is used at the larger stockpiles.
Groundbreaking for the chemical destruction facility took place on October 28, 2006. Final design of the facilities should be complete in 2010 and actual construction in 2018, after which destruction of the weapons will begin.
In March 2005 the design-build-operate-close schedule was extended to make the program more affordable on an annual basis. Site preparation work and the construction of support buildings continued and final designs for the remaining BGCAPP facilities were completed in 2010. After systemization of the facility, destruction operations are expected to begin in 2020
and to be complete by 2023.
The plant will operate until all the chemical weapons have been destroyed. Closure activities (shut-down, dismantling, and restoration of site) are slated to be wrapped up by 2026. This schedule exceeds the terms of the Chemical Weapons Convention.