Because it's not that easy. Otherwise Yasora would have done it back in the bad ole days when there wasn't anything like spellcards to keep people in check.
When would that have been exactly?
She died invading the moon, but the Lunarians also had Weapons of Mass "Purification" with power pretty close to that of a nuclear explosion, and Yasora didn't hate Yukari until after she realized the lunar invasion was a set-up. The very next thing Yasora tried to do when she regained consciousness was kill everyone in Utsuho's sleep, but Satori stopped her. The only time she ever had control between when she first wanted to destroy Yukari and the present is right now.
It's nuclear fusion. The easy part is blowing everything up, the hard part is containing it. Converting even .6 grams into energy yields a blast with strength between 13 and 18 kilotons of TNT. All it would take is an unstable mini-sun (which, left to themselves, couldn't possibly have the gravitational force necessary to counter the energy suns expel outwards), or even a mini-sun without the layers of spells Utsuho must use to keep them coherent, to destroy Gensokyo.
Heck, considering how much Utsuho's pestering Yasora, there's a good chance that one of the mini-sun spells could destabilize due to Yasora being distracted. It's a very good thing that Utsuho can't be annoying during the spell cards.
