Q:

Why only a single loopback address on IPv6?

IPv4 allocates a whole block of IPv4 loopback addresses, 127.0.0.0/8. IPv6, in contrast, only allocates a single loopback address, ::1.
This seems surprising that IPv6 would be so stingy in its allocation of loopback address(es), since overall it provides a huge increase in address space.
In IPv4 I've found it useful to be able to use multiple loopback addresses. E.g. for certain complex DNS set-ups, I can install two local DNS servers, one on 127.0.0.1:53, and another on 127.0.1.1:53. (It's a bit easier to refer to the different servers using different IP addresses, rather than using non-standard port numbers on a single IP address.) I can't use this strategy on IPv6 with only one loopback address.
What good rationales might there have been for only allocating a single loopback address for IPv6, unlike for IPv4?

A:

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).

