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.
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.