the main reason is space...

plenty of single turbo LS1's, LS2's etc.
in terms of air volume, they're pumping more, so turbo size isnt the problem.

Engine bay space is an issue for more street setups.
low mounting turbos typically has space concerns aswell, you don't want them hanging too low, for obvious reasons, coupled with the need for oil return/pump to sit even lower.

saying that, Mr CVE had his 3V, with a single mid mount turbo.
He's also done another 2 since, and all had results between 300rwkw and 370rwkw on relatively low boost.
I'll be doing mine soon, just sourcing injectors.

I also purchased a mid mount single Turbo Boss 260 setup last year, the 260 was installed in a BA Ghia.

So its been done, its just not overly popular.

a lot of the numbers quoted in this thread seem incredibly inaccurate.

A single turbo on a V8 doesn't mean just running it off one bank, There is a cross over pipe so the single turbo runs on both banks, there are complexities in piping obviously. but in terms of 'offer very little' theres no reason to assume that.
if the turbo is correctly sized for the application, it will be just as good as a pair of smaller turbos.