Speaking only for myself: Because Gerrit's unit of review is a single commit instead of a branch. Commits are units of change, but the unit of work and review is almost always a series of related changes. Just because the changes are individually atomic (testable, cherry-pickable, etc) doesn't mean that I want to review them that way. The typical flow is for an engineer to produce a cluster of inter-related changes that motivate and rationalize each other.

Reviewing such a chunk in Gerrit is a PITA. It breaks up the flow of discussion and encourages a myopic perspective on the work.