Because chemical weapons, and nerve agent in particular, besides being deadly, are very tricky to use. It is easy to inadvertently kill yourself if you are not careful. Additionally, the wrong environmental conditions (wind, water (humidity or rain), temperature, etc...) all impact agent effectiveness and duration (how long it remains lethal).
Most chemical agents were designed to be inserted into munitions that were either fired out of a cannon, or dropped from an airplane to target an area. Using them against a more discrete target, like an individual, is more complicated and more difficult.
Novichok, if we understand it correctly, was designed to be a binary agent, safe to carry until mixed for use. Which also increases the possibility of errors as it is prepared for use by mixing it incorrectly, or contamination by something that impairs agent effectiveness.
Any or all of the above could explain why the Skripals didn’t die.
Retired from THE Corrections in 2003. We had to have gas training every year. At that time, we had CS and CN gas. Most were for use with the gas gun, but we did have a hand-held aerosol that could be used in enclosed areas, for example, extracting a combative inmate from a cell. I have a cousin who works in the same system, and he told me that the state is going to be adding pepper spray to the arsenal.