thats because some people make the mistake of assuming that just because your tank hasn't actually fallen through the floor that everything is fine.All this talk about leveling tanks and then people here minimize the threat of warping floors...brilliant.
it may not be.
when I was house hunting, I knew I wanted a big tank. I ended up going with a 72x18x29 165g tank (that aqueon lables a 150).
I knew that to put it on the main floor would require reinforcement, especially given that I wanted a steel tube stand that my LFS sells that has 4 legs. the amount of weight per leg is huge.
I eliminated the issues by finding a nice house that had a finished basement on ground level.
but I still maintain that if your tank is 150g + the best place is ground level.
not just because of weight.
if you ever had a flood, water and gravity work together to destroy the floors/walls below your tank if its on an upper floor.
so ground level is always better for big tanks. always.