From what I've read, the mushroom cloud from a 'nuke' is the same as one from a firebomb.

And also, buildings that shouldn't have been standing afterward, were, like a church, at the center of the bombing. People inside survived. If it were a nuclear weapon that shouldn't be the case. It's really the most damning piece of evidence.

On the circumstantial evidence front, the u.s. has a history of big lies, was in a life or death struggle and so had motive to lie, had the means to spread such a lie, and there were, early on, conflicting reports of nuclear attack versus firebombing.

For one link discussing the idea, see http://www.renegadetribune.com/hiroshima-firebombed-not-nuked/

Why shoudn't it be?

Either you know and acknowledge the truth or you're wittingly or unwittingly working for the enemy.