I imagine it could have something to do with its intended use by mounted (cavalry) troops. In very early manuals for the 1911/1911A1 pistols, the slide locking function of the manual thumb safety (called the "safety lock") is specifically mentioned, so it seems to me that folks of that era considered it a feature, not a bug. An early copy of a 1911 military users manual from the Ordnance Department can be seen here:
Description of the automatic pistol ... - Google Books
Check pages 10, 13 and 14 to see the references I mentioned above concerning the safety lock "positively locking the slide" closed.