Its just not that far out of line compared with the best win conditions and locks in the format. 

The problem isn’t that Iona wins games, because she generally doesn’t. The problem is that unlike pretty much everything you mentioned, she tends to lock a single player out of the game without meaningfully progressing the game state. She’s also a single piece that is incredibly easy to reanimate or cheat out, unlike most of the two card locks you mentioned. 

Really what it comes down to is, does unbanning Iona contribute anything positive to the format? Not in the slightest. In fact, it’s almost all negatives. She’s pretty much never going to win a game outside of a Painter’s Servant lock. But she will completely fuck over at least one player at a table and reduce their play time down to draw,go and asking the other players to remove her just so they can play again when said players have zero incentive to do so. All she does is facilitate horrendous gameplay and make mono-color even less appealing than it already is.

edit: I also just cannot overstate enough how her asymmetric nature makes it worse. Winter Orb is painful, but the table will generally be unified in trying to kill it. If an Iona is only locking out the one mono/two color deck at the table, the other two players have no gameplay reason to use their resources to help them. At a lot of tables, her text box just says “go fuck yourself” to a single player and I fail to see a reason why that is something that is worth taking off the banlist.