End of turn draw chance is massively superior to beginning of turn. He is op and an auto include at 2 mana draw and end of turn; and he's not playable at 2 mana draw at start of turn. Think of it like this, warlock hero power is borderline overpowered and it costs 2 mana AND 2 health. Draw is ridiculously powerful in hearthstone.

Considering this weird spot of totally op vs totally useless at 2 mana, we need to find another way to make his ability good while not making him auto include in all decks. Adjusting his mana is the best place to start.

3 mana, draw at end of turn means you get a chance at drawing a card, which means your mana, and turn, isn't completely wasted. However, because you're getting guaranteed, immediate value from Nat he can be raised in cost, which allows for the opponent to deal with him better. 3 mana to play is a tempo risk, while 3 mana to counter isn't as rough of a tempo loss to clear.

There are a lot of reasons why 3 mana 0/4; 50% to draw at end of turn is much superior to 2 mana 0/4; 50% to draw at start of turn.