Because it would have been less-effective, I expect. The targets were chosen because they had military industry and had not yet been destroyed via conventional firebombing, which had already been done at mass scale in other places.

I think that it's important to understand that the atomic bombs were simply seen as something of a significant multiplier in the existing bombing campaign. One bomber with an atomic bomb could maybe do what a thousand bombers with conventional weapons might...but there were, in fact, thousand-bomber raids happening. That is, cities were already being set afire. The Manhattan Project simply permitted doing so with a significantly-lower resource expenditure.

EDIT: Also, to be clear, the US fully intended to ramp up to mass production and employment of atomic bombs, dozens a month, once production could be brought up, and would have done so had the surrender not occurred.

Today, partly because of (significantly more powerful) thermonuclear weapons and because we know that the first two bombs did result in a surrender, the first two atomic bombs maybe look like something of a clear bookend to the war, but that's for us in 2023; in 1946, they would have been another step -- if a significant one -- of World War II's large-scale bombing campaigns, something that had been growing for years.