Because they lack gear checks, and too many people who lack gear will try and get in it for the better loot without having appropriate gear for it because they think people who are geared will just pull them through.
By removing it from the group finder, you're penalizing people that use it with premades.
Feel free to not use it for pugs, that is your prerogative.
And Lost Island is a SKILL check, not a GEAR check, anyway. Give 4 unskilled players a complete set of black hole/campaign gear and they will still wipe on LR-5.