Twitter 2FA

Maybe this has been addressed here before but I couldn't find anything on it, so here it goes. With all the stories of hacked Twitter accounts, why does anybody know why they don't have a better implementation of 2FA? For example, I have two Twitter accounts I manage (one personal and one business). I can only implement 2FA on one of those accounts because Twitter only allows one account to be associated to a phone number. I thought I was going crazy but a Google search turned up numerous people complaining of this very issue. So does anybody know why they don't have a better implementation of this?