Only a few countries such as Russia and the US really ever cracked the technology for making “unitary” Sarin that had any kind of useful shelf-life. The main chemical reaction that produces Sarin creates 1 molecule of hydrogen fluoride, a potent and dangerous acid, for every molecule of Sarin. This residual HF destroys nearly anything the Sarin is stored in, and quickly degrades the agent.  The US and the former USSR made a huge effort to find a way out of this problem.  They found different ways to refine the HF out of the Sarin using very expensive chemical engineering techniques. 

Furthermore, The Organisation for the Prohibition of Chemical Weapons inspections after Syria’s accession to the Chemical Weapons Convention in 2013 revealed a variety of fixed and mobile mixing apparatus for making binary nerve agents.. Syria either did not develop such techniques or decided it was far cheaper, safer, and easier to stockpile binary components for a “mix it as you need it” process.