[bdarnell]

(disclosure: CockroachDB founder) The reason I've heard is that Spanner uses a separate mutation API instead of SQL DML because of a quirk of its transaction model. Writes within a transaction are not visible to subsequent reads within the same transaction (source: https://cloud.google.com/spanner/docs/transactions#rw_transa...). This is different from other SQL databases, so the use of a different API forces you to think about your read/write transactions in a Spanner-specific way.

(FWIW, CockroachDB does not have this limitation - transactions can read their own uncommitted writes, just like in other databases)