Somebody doesn't do any address or route planning.
In IPv6 the amount of hosts in a subnet is totally unimportant (because there are always 64 bits for this). If you have, say, a thousand hosts you're going to need to buy decent network kit 'cos a pile of daisy-chained 5 port plastic home switches won't like that - but it's only a local problem, like buying enough cable. You can have however many subnetworks you felt was appropriate for managing and organising things, and only those need managing. However in IPv4 you need to know how many hosts there will be or might be in each subnet, in order to plan address allocation, and small changes can throw things into turmoil, you have to manage the individual host addresses.
Suppose I have four subnets with 40-50 hosts in each - in IPv4 chances are that's four /26s. And then somebody wants to add 20 hosts to one of the larger subnets so now it won't fit in a /26 any more. Ugh. This is likely to involve a re-numbering programme that might take weeks or months. I may need to reach above me, to find somebody who has enough address space to trade with me, and they may in turn have to reach up too, or worse find the money to buy space. Suddenly what should have been an easy problem ("add twenty new hosts") is a nightmare with a budget and project management.
IPv6 evaporates this entire class of problems. There might actually be people at large organisations whose job ceases to exist under IPv6. Certainly there are people whose job gets much easier and less stressful, and who don't have to say "No" as often any more.
Do you have examples of software that you can't use because it doesn't support IPv6? Of all the software I've used there isn't any, which is why I'm curious.
is this not trivially fixable by running software that tunnels IPv4 into ipv6 between hosts who want to play multiplayer?
I have seen some implementations of IPX which work by tunneling IPX inside IPv4.
On top of that, there's a whole lot of software that either doesn't support v6, or has major problems.