Paladin Bard gets a bigger hit die and better DC's than paladin wizard.
What you get with paladin wizard is rituals. That's the reason you would take it over bard.
Or you just like the idea of a paladin with a book.
Either way. Why make them pay more?
Not sure how Con plays into anything.
Dex based classes can combine just fine already. There is no
extra cost in being a ranger/monk/fighter/rogue/druid/cleric.
Or you could be a paladin/barbarian/bard/fighter/sorcerer/warlock without paying
extra stat cost.
What's the point of drawing the line at paladin monk?