Not for lack of trying.
https://en.wikipedia.org/wiki/Assassination_attempts_on_Fide...
Like I said below, when the largest empire the world has ever seen makes it a policy to destroy any socialist leader (even the mildest of social democrats) that threaten their resources or hegemony, only the iron-fisted strongmen survive. What people cite as failures are a result of the intense external pressure which favors strongmen because democratic socialist movements get massacred.