Because it already was a complete deck, it's just the stock Midrange Hunter deck from last week with less removal and add 2 Mad Scientist/1 Leeroy. Mad Scientist is just that good, the ultimate tempo play.Reynad's Mad Scientist Hunter already looks like a deck polished by a month of testing. It's complete as is.
Deathrattle Hunter beats it consistently, really any Face strategy does - but unless you play against nothing but slower Hunter decks your in trouble. That's why both Trump and Amaz switched to it after trying to counter it.