I don't know the team specifically, but I suspect the difference comes down in part to Go open sourcing early in development, thus finding a bunch of the rough edges that exist outside Google's walled garden early in the project's life when there wasn't much compatibility _to_ break. Blaze was a mature project within Google for years before Bazel was opened up. Many of the breaking changes seem to be taking one-offs built for features within Google (e.g., the handling of protocol buffer rules) and building those in terms of more general and composeable features.
The net result is a Bazel (and Blaze) that are less burdened by the baggage of legacy, but the cost is a faster treadmill to keep pace with changes.
The net result is a Bazel (and Blaze) that are less burdened by the baggage of legacy, but the cost is a faster treadmill to keep pace with changes.