Objects in a scripting language are a meta-Universe each. In the abstract, operators must be considered in light of these. The designers of C appear to have recognized that equality was not to be taken with levity; that a comparison operator is not the same thing. And thus chose ( a == b ).
To leave the single symbol unused would have meant lengthy explanations to new programmers.
This might give you somewhere to start, if you're interested in knowing what has been going on for the last 50 years.