The Metalic dragons wished to help the humanoids with their troubles, while Tiamat forbid it and the chromatic dragons agreed.
Tiamat forbid it for 2 reasons. The reason they told everyone was the partial truth; if they helped humanoids with every issue they had, they would never grow stronger and smarter.
The secret reason which was even more true, was that Tiamat despised mortals, and found their weakness sickening. The idea that a race of gods would debase themselves like that sickened her.