Probably related to decompression time and air bottle limitations.
The deeper you dive (and the more times you dive) your time on the bottom gets shorter due to decompression requirements.
And your air bottle has to supply you for the time on the bottom plus the decompression stops on the way up.
We'd need decompression tables that I don't have handy to see what's required for a 40 minute stay at 60 feet, plus what the current "sport diver" bottle holds, and then allow downward for inexperienced divers who do not use their air efficiently.
I checked. The dive table says 55 minutes at 60 feet. I guess that makes sense but if you used a double tank and allowed for decompression you could stay a lot longer.