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.

I heard that the US Army ask for a safty on the test pistols. They also wanted the bullet weight increased from the 200gr to the 230gr.

Thanks guys. The manual does mention specifically locking the slide, but doesn't really say why it needs to be so.

Maybe a jarred slide can cause enough inertia for the firing pin to do its job?

Originally Posted by ScottChapin

Thanks guys. The manual does mention specifically locking the slide, but doesn't really say why it needs to be so.

Maybe a jarred slide can cause enough inertia for the firing pin to do its job?

Yes, if you drop an old-style 1911 on its muzzle from a sufficient height with a round chambered, it will fire. However, it will do this whether or not the safety is on or the slide is locked, so I don't think that is the case. By the way, this was the reason Colt added the firing pin safety to the 1911A1 design (series 80s guns).

There may have been concerns about the slide being pushed back out-of-battery just far enough to prevent firing, perhaps during holstering or CQB/mounted operations. The slide locking function WOULD prevent that type of stoppage, which can sometimes be very difficult to detect ahead of time.

Here's the reason:
The Colt's Government Model's safety locks the slide to make reholstering simple for the cavalry trooper, who was limited to using one hand. It also makes reholstering easier for us, without the one-hand limitation.
When you reholster a pistol, the slide of which is not locked in place, you have to pay attention to making sure that the gun's slide remains fully into battery while the pistol is fully inserted into its holster. In the case of the Colt's/Browning design, one need not pay full attention to the task. Merely shoving the made-safe pistol down into its pouch does the job.

Originally Posted by Steve M1911A1

Here's the reason:
The Colt's Government Model's safety locks the slide to make reholstering simple for the cavalry trooper, who was limited to using one hand. It also makes reholstering easier for us, without the one-hand limitation.
When you reholster a pistol, the slide of which is not locked in place, you have to pay attention to making sure that the gun's slide remains fully into battery while the pistol is fully inserted into its holster. In the case of the Colt's/Browning design, one need not pay full attention to the task. Merely shoving the made-safe pistol down into its pouch does the job.

That's great . That makes so much sense. It's one of those; "duh, why didn't I think of that?" DJ Niner too, basically the same answer.