[kbolino]

SLAAC requires the bottom 64 bits to be part of the host portion of the address. A network prefix larger than /64 limits SLAAC to providing link-local addresses only, which means another mechanism needs to provide routable addresses, such as DHCPv6. That, in turn, prevents the use of privacy addresses.

[vetinari]

DHCPv6 is also optional, clients do not have to support it; some do not support it intentionally. So for example, any Android device won't be up and running on SLAAC-less network.