The sidelock has much more in the way of moving parts, Larry, and needs more maintenance. I'd bet those "keeper's guns" we see from time to time didn't get that maintenance, either. A lot of still functioning boxlocks in that catagory.
I got a sidelock that was just worn out. And, I still see lots of them, in that same condition, on websites and in dealers rack's, today.
I guess if I had been a newly minted English officer, in the India Corps, circa 1906 or so, I know which design I would have picked for that duty.

Best,
Ted