The reasons are always the same: money, power and control.
From this article on Wikipedia https://www.quora.com/Why-does-the-US-want-to-unseat-Assad-in-Syria
“In July 2011 Iran, Iraq and Syria said they planned to sign a contract potentially worth around $6bn to construct a pipeline running from South Pars towards Europe, via these countries and Lebanon and then under the Mediterranean to a European country, with a refinery and related infrastructure in Damascus”
“On 29 July 2011, seven defecting Syrian officers formed the Free Syrian Army (FSA), composed of defected Syrian Armed Forces officers and soldiers, aiming "to bring this regime (the Assad government) down" with united opposition forces”
USA wants to get Assad on the charges of mass genocide as well as human rights violations. But this is happening in many other countries in the world like South Sudan, Congo, Yemen etc., and US has turned a blind eye to them.
