The problem with making CoTC scale with enemies hit by hard CC is that it directly reduces how good it is on tanks that can only CC one person.
As it stands now, Maokai can dive into the enemy team to root the carry and still get a good shield so he can last for his team to arrive. With the change, he can't really do that. This hurts his ability to single targets out (though he's a warden tank last I checked so I'm not sure if he should be balanced around that).
Though if the CD change doesn't work, I think Riot might try having it scale with enemies hit by hard CC. Though doing so makes it more niche and causes AoE CC tanks to save it for when they'll get the most out of it, which may cause them to peel less.