Mario Party 7 has these individual board runs in the level leaderboard, so why not Mario Party 6?

Because it's really arbitrary, there has never been any interest in doing them for the last three years that I'm aware of, there's also the problem of even trying to define a proper ruleset for those because of all the customization you can make with mini-games and settings.

Instead I let some of the mini-game have a reserved place for the IL board which seems to have some small community interest.

Speaking of that, I saw that another thread of yours spoke of another mini-game that is not on the list. Was that a mic exclusive mini-game and if so, how many of them are there because I would not mind adding them.