For the 1911, the Brill doesn't cover the trigger (it covers the guard, but is cut away to access the trigger). Same with the revolvers: the triggerguard is used to position the gun, and to retain the gun in place, but the trigger is exposed. The Brill was designed in a different time, for a different target market. It wasn't designed so much a "concealment" holster as a less-obvious holster for folks who were intended/required to be armed (i.e., Rangers and general law enforcement) and expected to need that pistol in a hurry. They considered the exposed triggerguard an acceptable risk.
The design is focused on function before concealment. It gave LEOs a functional fighting holster they could cover with a coat when in court. IIRC, at the time it came out/was in production, Texas didn't allow the carrying of pistols by private citizens when off their property.
Most CC holsters do cover the trigger. I suspect a big part of this is perceived liability, that if the trigger is exposed and the gun discharged, someone somewhere will argue that the holster designer or maker is liable for the ND. Part of it also is that times have changed. Most folks carrying a pistol today do so in case it is needed, rather than with the expectation that it will be needed.