They're different fighters. If Holly was more aggressive she would be at greatly increased risk of getting taken down. She is a counter fighter, a damn good one. Amanda Nunes is an aggressive fighter, a damn good one.
For Holly to perfect an aggressive striking style, which keeps someone like Miesha on the end of her punches, while staying at a perfect range to defend TD, she would have to change the way she fights/trains considerably.
This is the exact reason that Holly looked amazing against Ronda, but not so amazing against Tate & Pennington. Ronda played right into her hands by charging forward.
It's not as simple as Holly being "more aggressive"