Because, in the early days of Unix systems actually being used as multiple, simultaneous, user systems, you might have one group of users collaborating on a project, and they would have a shared directory (via the 'group' owner) where they would store shared items. Each user would create various files, and each file's space consumption was charged to that user, but the shared directory might contain multiple files each owned by different users (but all owned by the shared 'group' identifier, so the group could access them).

For a group shared directory, assigning the disk space usage of files therein to one single user (ignoring the aspect of "which single user do you pick") is unfair to that user (his/her allowed maximum disk space is consumed) while everyone else is not charged for their actual usage.

This all came about to try to enforce rules to prevent one (or a few) rogue users from using up all disk space on the system for themselves, leaving no one else with any disk space available for their own usage.