Most often, it's a "We built this thing initially in X until the legacy technical debt and proto-duction compromises caught up with us, then we rewrote it in Y,

Most often, it's a "We built this thing initially in X until the legacy technical debt and proto-duction compromises caught up with us, then we rewrote it in Y, leveraging all the domain knowledge and experience we've gained after doing it in X. Amazingly, the second system in Y is better/faster/has less bugs!"