We rely on achieving cross-zone quorum for many operations internally, with the aim being to guarantee the durability of data if a zone is lost. Doing this means that we need to operate across at least three zones: under a two-zone setup, losing a zone would mean that we may no longer have at least 2n+1 nodes available.
Until very recently the London AWS region only had two zones, so it was unsuitable. Now that it has three zones it’s viable, but we don’t have any short-term plans to move to it. 