Smite had already flown towards the ancient ice titan equipped with a sword and shield. This left them little time to choose, so they agreed on the ancient earth titan.
It is the golden one with four bladed arms. The tree titan is also slow, stable, and grounded, but it has too many arms to attack with. By the looks of things, it looked like it could fight an army so they chose to avoid it.
Even Smite who could probably use fire against the tree avoid it in favor of the ancient ice titan. All six of them rushed towards the golden ancient titan with a single eye.
The remaining gods also left behind the hunt for divine calamities in favor of killing ancient titans in order to acquire fragments of their hearts to create a hero.