Gerrit is so off-putting to new users that many never get over the learning curve. At Wikimedia we want to be welcoming to new contributors. We also have to consider the on-boarding experience of new staff members, as well as the productivity of staff and long-time contributors. Gerrit satisfies some people who have used it for a long time but it is almost universally disliked by newcomers. When your users are volunteers, you can't force them to use Gerrit until they get used it it. If the experience is bad enough then they will choose to spend their time on something else instead.