Why is there not many co-op categories other than like arachnophobia

The main co-op categories are tracked here on haloruns, which is the easy and legendary difficulties: https://haloruns.com/records/odst/fullgame/co-op

While I do agree that there is a lack of co-op categories (mainly for mombasa streets), they are most likely not listed on the leaderboard for the sake of not having empty categories. You can always run whatever co-op category you would like and post your run in the forum, and perhaps the category would be added that way