They are very common in Australia. IDK about elsewhere.
I think because its easy to keep a stash of pre-filled gas bottles in the average small warehouse, as opposed to a drum of smelly diesel and some way to pump it.
They're popular in Canada too. One of my previous workplaces had a large supply of propane on site for other production purposes and we could refill the forklift tanks anytime.
I'd guess a propane tank. I don't have any experience with forklifts, but I know there are lots of propane powered ones (in addition to the electric ones)