The reason why this account was locked is missing from this request. For an analogy, fetching userinfo optionally returns the block reason if the user is blocked.
Description
Description
Event Timeline
Comment Actions
On a technical level there is no such thing as a 'lock reason'. The best alternative is to query the comment field for the last globalauth log entry for the user, for example https://meta.wikimedia.org/wiki/Special:ApiSandbox#action=query&format=json&list=logevents&formatversion=2&letype=globalauth&letitle=User%3A%D0%A1%D1%82%D0%B0%D1%80%D0%B8%D0%B9%20%D0%94%D0%B6%D0%B5%D0%BD%40global.