Why couldn't they just build another throne for Hestia instead of kicking her out?

Because if they added a new throne for Hestia, the 12 Olympian Council would become the 13 Olympian Council, and in Greek and Roman Mythology, 13 is a very unlucky number. That's why they didn't add another throne.

there can only be 12 major gods 12 thorns you know 3 is a good # 12 is 3x4

it wood unbalance the council

because it's traditional to have twelve seats of power