It's because of MAD.No it's not. You might think that the Dex/Wis cleric is the best - but Str/Wis is only viable with heavy armour - and heavy armour breaks pure Wis clerics into two pathways.
Medium armor is only 1AC lower assuming 14 dex and con.
Heavy armor is probably giving up 5' movement which matters.
But the kicker is as you level weapons becomeess and less relevant due to cantrips scaling. If you buff your wisdom your cantrips and spells like spiritual weapon and guardian are better.
AC also scales more than saving throws.
The heavy armor cleric gas to make a choice between being slower, being less accurate if they're using weapons or buffing wisdom.