Even with slugs, the shotgun has limited range when compared to nearly any rifle or carbine. Buck has limited range, and its ability to penetrate common barrriers is limited too. Additionally, the shotgun is more gun than some can handle, most examples are longer and heavier than a carbine, and they kick plenty as well. And a shotgun has a unique firecontrol/manual of arms that is not common with semi pistols or carbines.
The role of the shotgun, even in LE, is steadily shrinking. Many agencies now issue carbines.
I would take nearly any long gun over a handgun, including a shotgun. And I'd hope I could get slugs for it. But given a choice, I'd take a carbine or rifle ahead of a shotgun.
For HD, a shotgun with buck is still plenty, but for LE and military, the shotgun now plays a narrow role.