Because she had her foot on the brake while the car was in gear. Which is why she was stopped. If she had woken up and taken immediate direction from the officer, to put the car in park- then that would be been that. Or if she had removed her foot from the brake at any time, the car just being in gear would have caused it to potentially roll forward since it was still in gear. He was trying to stop that. What he didn’t account for was her not immediately taking direction when she woke up; and instead she didn’t just move her foot from the brake, but actively put her foot on the accelerator causing it to not just roll forward- but to travel forward.

The officer was low on options here. If they had a secondary unit on scene- the secondary car would have parked directly in front of her car to keep her from doing exactly that. Instead- this officer was more or less forced to leave his car BEHIND her car to maximize their visibility for cars that were approaching.