They wanted to make them more focused
The lack of an untap phase keeps this card in check without completely destroying its benefits
He has a very high ink cost of 7 for summoning him, but that high cost is worth it as he has high stats and incredible abilities
Junkyard Genius BPhillipYork: There's a lot of ETBs making powerstones, which can get abusive, but handing out menace isn't particularly meaningful, haste on the other hand is a powerful enabler, and obviously this guy creates his own token to eat, in decks that want to sacrifice or create artifacts and maybe part it with ETB or death punishments for artifacts it can be a useful piece
Trample and Deathtouch together mean it really doesnt matter what your opponent has on board
Niv-Mizzet, Parun Next up we have none other than Niv-Mizzet, Parun himself