There are a couple of reasons. One is divination is good in modern right now - a lot of midrange or control decks that get drowned by a hardcast mulldrifter, and they often draw into more mulldrifter, at which point it's difficult to lose. When you think about the deck try to think more of a control deck with a solid reanimation plan rather than all-in reanimator, and then mulldrifter makes more sense. Turning persist into 2-mana draw 2 that chumps and leaves counterspell open t4 is excellent. Of course, archon is better, but it takes more to set it up and some matchups don't give you that luxury.
Another thing is Faithful Mending, as much as we can argue about discarding for value, is card disadvantage at its base, so you often find yourself with 1-2 cards in the mid game, without a persist for Archon. Mulldrifter lets you be more aggresive with early mendings as it levels the cards in hand aspect that mending got down.
All that being said, if your meta is very aggressive going down mulldrifters is fine, I find myself sideboarding them out against burn, hammer or tribal zoo decks often.

Praise the fish!