From all the tents I saw, including the two I have, most of them had black floors. Sometimes, the older tents have differently colored floors, but all the new ones are all one color (usually bright orange/red/green), except the floor - it's black. Why?
One reason (and I don't know enough to suggest this is the main reason) is for identification.
When putting a tent up you may have a bundle of fabric that you need to set down on an uneven surface, sort and erect. Having it in black distinguishes it from the varied colours of the tent walls so you can always place your tent floor down, without having to check all the sections. The floor is the strongest, most heavily waterproofed part of the tent skin, so this also means you are less likely to end up with a damp tent from placing it upside-down on damp ground, or tears from catching branches or roots on a thin piece of tent fabric.
Found another reason:
Because the floor is manufactured using a different process to the rest of the tent, making floors that matched the tops would require more complicated processes and cost more. A constant black means any floor can be fixed to a top of the correct size.
For the most part, the color of the floor is ambiguous. While most newer tents have the same color floor, it's simply a matter of cost.
Dyeing plastic increases the cost, as black is the most simple natural color to produce.
The material is different however only in the sense of durability, moisture wicking qualities, and weight. While it's thickness and material does provide some protection against the elements, it's practically none.
Why do most tents have black floors?
Because it's the cheapest acceptable standard within the market. (Consumer happiness Vs. Competitive pricing).
A quick online search among performance tents sold by a major U.S. retailer and a major U. S. based manufacturer showed no tents with black floors. Many had grey floors, in various shades, which may be what you meant. Some tents also had floors with colors that matched or coordinated with the tent body or the fly. My personal experience is that I've had both grey and matched colors on tents. Also IME, low performance tents sold by mass merchant stores are more likely to have color matching or coordinating materials on the floors.
I suspect that grey flooring material is a fashion choice. It is a neutral color to coordinate with the various colors used on the tent body and fly. The same grey is often used for netting, trim and pole sleeves in my search. It also is less likely to look shabby from dirt, stains and other wear of use, relative to more colorful fabric.
The sharing of one floor fabric color among many different colors of tents is also likely to be an cost saving choice that can be made without impacting the fashion appeal of the tent.