Is there a reason why lower voltage batteries are preferred for high AH?
They are sold by the cell.  

You assemble cells into batteries.

Common word usage in English has blurred the distinction between "cell" and "battery."

A cell is the smallest construction capable of generating voltage.  That voltage is dictated by the chemistry.

A battery is a bunch of cells in series used to generate a voltage higher than that of a single cell.

The cells you were looking at are intended to be used to build very large batteries.

You connect them in series to get the desired voltage, then in parallel to get the desired current and redundancy.

You can then replace individual cells that go bad instead of replacing a larger battery.

Another, final point to consider is weight.

The cells you were looking at weigh over 400 pounds (over 200kg for you metric types.)  Easier to work with individual cells than a battery that would weigh over a ton.