Why only a single loopback address on IPv6?

They are not needed. Simple like that.

You can always assign link local or private addresses as you need to a loopback adapter - in the rare case you really do that.

I would say that most people are not even aware you can use anything else than 127.0.0.1 an you are the first person I have met in around 20 years claiming to use more than that. Not joking.

Most multi server setups do not need it (HTTP host header is used instead - and yes, I know this is limited to http, but I would say most servers needing this are from web developers).

If you want more addresses available locally, you can just assign addresses from a ULA range of your choosing and assign them to the local machine (on the loopback interface, if you like). That'll give you all the addresses you want. You don't have to be granted a special block from which to assign addresses that your machine will respond to locally.