As we all know, naming things is one of the two hardest things.
Just call it something like "toxic gender roles" or "toxic gender expectations" and you short circuit the entire debate.
But regardless, the name does derive from some historical reasoning. For many decades feminist theory used to consider the feminine to be aberrant and the masculine to be the cultural norm and a pure good, only investigating the ways in which femininity trapped and harmed people. "Toxic masculinity" was a re-evaluation of these assumptions, identifying ways that masculinity, like femininity could trap and harm people. The term was positioned against the assumptions of the time and "toxic masculinity" expresses this position much better than "toxic gender roles" since it was explicitly about changing how feminism thought about masculinity.
That is no longer especially significant to the term, but inertia exists.
Feminist theory is a serious academic discipline that isn't just making shit up to attack men.