You are either running an emulator or the RNG just hates you. The natures for the starter Pokemon are not predetermined or locked.
As a matter of fact, I started a W2 playthrough a couple weeks ago, and I wanted to use Snivy. I soft reset for a good nature, because Snivy isnt that great. I got different natures every time.
The best way to get the nature you want for this Pokemon is just to soft reset for it. There's no other way to obtain another Unova starter until after you beat the game, (the Daycare is post game, which is so ridiculous.)