How do you describe a particular genre? The one obvious thing this particular kind of games had in common when they appeared was the .io domain, thus the name.
Go to cursors.io, agar.io and slither.io. Now find a short phrase to describe all three.
A genre is precisely a collection of artistic attributes that occur and are repeated mostly as a historical accident (usually after a pioneer explores the space and follow-ups revisit) and are shared among a collection of works -- they are hard to concisely label with precise descriptive terms so their audience seeks a fairly arbitrary label to describe the collection. "io game" seems reasonable enough to me.
Just the same as (made up example) "Up-tempo minor-seventh-key highly-distorted guitar arpeggios with four-on-the-floor beats and legato vocals" gets called "wailcore" or something, the kids playing these aren't going to say "HTML5 games with an emphasis on large counts of concurrent players, with gameplay arising from emergent behavior of players instead of a large investment in story, art, or game mechanics." a hundred times to their peers -- even if they could consciously notice these attributes and articulate them that way.
They'll start saying "you know, it's kinda like agar.io" "I haven't heard of that is it like <whatever>.io?" "Yeah, exactly" suddenly you have a community of players of "io games". I don't really understand how anyone would expect anything else here.
Also it's just a bad name in general, since those style games can be hosted anywhere.
"Rock" for instance.
