Because past ideas have shown how abusable doubling the amount attacks would be regardless of afflicted creature's stat [Adrenaline subverts this by effectively cutting down the damage done by each successive hit and by cutting down the amount of attacks when attack goes up]. Without the restriction, self-buffing gets out of control.
And it was supposed to help make use of shields' secondary effects more, acting as a subtle buff to Fire Shield, I suppose.
I chose to make it counter Adrenaline purely for sake of making an environment where it could not be used with Adrenaline, which I felt would add an unnecessary layer of complications.
