how would you do SLAAC with 64 bits?
the goal being to support a model where one could support multiple prefixes to handle the common case of multiple internet connections. more importantly to allow providers to shuffle the address space around without having to coordinate with the end organization. this was perceived to be necessary to prevent the v6 address space from accruing segmentation.
Keep in mind that SLAAC isn't. Modern IPv6 stacks use privacy addresses, so they still need to run the address collision detection.
There's also a proposal to have SLAAC with longer prefixes, because otherwise you need to use DHCP-PD if you want to have subnetting in IPv6.