First of all, good question. Second, this really is more of a discussion topic.
But I would like to give my two cents to the topic.
Does having dark background and white text impact usability?
No. If you put that layout through a web accessibility color contrast test, it would pass. But there's more to a layout than pure logic. Aesthetics come into play. That leads me to your second question...
Does it makes harder to read?
It certainly does. Black (or darker colors) absorb light themselves and from their surroundings. That would mean a loss of contrast of the foreground leading to a grayish feel even to
#FFFFFF colored text.
Now coming to the meat of the questions ...
Is there any specific reason to NOT do so?
There isn't a specific reason as such but any good UI designer would want the focus to be on the foreground. White is the most neutral color which makes the human brain skip it (like it was reflex) and jump to the next color visible. Hence any color that crosses the horizon of dark colors becomes a clear target for the viewer's eyes.
I hope this helps