It’s not so much the brute power, more the potential to unbalance the world.
While Ashbringer is probably more powerful in terms of brute strength, Frostmourne has the potential to destroy entire worlds with its power of corruption and domination, while also simultaneously corrupting the wielder to continue down that dark path.
In a 1v1 yeah Ashbringer wins, but in terms of which weapon is safer to give to a random “hero” who might have questionable motivations, it’s Ashbringer. Frostmourne has too much potential to influence the wielder to destroy everything.