Well, in the case of WoW, most players are attached to their characters because they've invested a lot of time in them. Most people have a "main" character that is the most powerful, the one they most enjoy playing, and they may have one or more "alts" that they typically spend less time with, and those characters are thus less powerful.

But this feature meant that through random chance, your alt could become much more powerful than your main. This sucks, because you're now incentivized to spend your time in the game playing a character you don't enjoy as much as your main, and the time you spent on your main making it powerful is simply eradicated.

And that increases the chance of you quitting the game.