When you discover a card it helps to be able to play that card same turn late game. That's a whole lot harder when the discover costs 5 mana.
5/3 stats is worse than 4/4 also.
Late game, the body is irrelevant. At that point you're just playing a card in the hopes to get a worthwhile card. Cards are best when you get full value out of it. A 5/3 I think is more likely to achieve that, and 2 of the classes have resource tools (innervate, counterfeit coin, preparation) to offset the cost of the minion or to create powerful swing turns. And 2 of the classes have powerful cheap spells, being shaman and rogue (huge list between backstab, eviscerate, shadowstrike, lightning storm, maelstrom portal, lavaburst).
I actually don't think it's that important to being able to play the discovered card the same turn the minion is summoned, but late game I don't think it's hard to use a 5 mana minion and a discovered card if you think that is important. Mid game,sure, but late game? Not so difficult. I think it's just a good on curve minion, unlike the other two which don't even trade with most if any played minions in their mana slots. I think the only upside those statlines have is easier to brann.