Well, you still need a way to manage them. Having an administrative web interface accessible over IPv4 but not IPv6 was probably considered Good Enough by whoever wrote the firmware.
Same reason cable modems don't mess with anything above L2 but are still reachable at http://192.168.100.1/ (no IPv6 equivalent).
That's a link-local address range (like 169.254/16 in IPv4). You'd need to specify which local link to use in order to connect to it (e.g. http://[fe80::aa:bb:cc:dd%eth0]/) and it wouldn't traverse past a router so wouldn't work if you have your own router between you and your modem.
The closest analog would be something in fc00::/8 (which belongs to fc00::/7 which is the IPv6 analog to 10/8, 172.16/12, 192.168/16), but good luck getting cable modem firmware vendors to all agree on which random address to use within that and then actually implement web administration and diagnostics on it. That's what I was getting at by saying there's no IPv6 equivalent; they haven't done that.
Same reason cable modems don't mess with anything above L2 but are still reachable at http://192.168.100.1/ (no IPv6 equivalent).