Out of Universe
Because the episode was about Iris
"I think this episode clearly shows that having Barry's powers for an episode really made [Iris] realize what she's passionate about," Candice Patton told reporters after a screening of the episode. "She understands that Barry having speed is what gets him up in the morning…and I think it reignited for her that she still has to find that, and that journalism is a thing she has kind of put to the side to take up the mantle at STAR labs, but it's still a part of her and it's still the reason that she wakes up."
At the end of the episode, Iris was quick to get Barry's powers back to him, with no real intention of possibly keeping them. Instead, she went back to writing about the Flash, bringing back that journalist side we've all been missing. Executive producer Todd Helbing calls her return to her calling a "reawakening," and said this episode felt like the most organic time to return to Iris doing what she loves.
Because making something cold is not the same as putting a fire out....they put out fires with water...not ice.
Regardless of the "science" though, we've seen in the past that Killer Frost using her abilities takes a toll. Putting out a FIRE TORNADO, even if she could, would likely kill Killer Frost.