It stopped due to politics, specifically LOL arguments about "inelegant developer aesthetics" coming from Mozilla https://hacks.mozilla.org/2010/06/beyond-html5-database-apis.... Somehow everyone and their dog using same Chrome codebase is fine, but oh no not the SLQ backend!
Instead we are saddled with garbage IndexedDB so bad nobody uses it natively and thousands of developer hours are spend building shims and wrappers to shield us from the brain rot.
But back then it wasn't a Chromium monoculture yet – Presto, Trident and KHTML were still going strong.
That the commitment to multiple implementations is something the WHATWG takes much less seriously than the W3C once did is another matter.