Afaik, to fill a gap in the hunting laws of some US states. Several states traditionally mandated “shotgun only” for hunting white-tailed deer. Recently, some of those states allowed the use of short range rifles too, essentially rifles in pistol calibers like the .44 and .357 Magnum. They did not name legal cartridges, but set up some arbitrary specifications for legal deer cartridges:
The cases must be straight taper. So all bottlenecked cases are out.
The cases must be shorter than 2”. This outlaws many existing straight cases like the .38-55, .45-70, .444 Marlin, .375 Winchester and old European or British cartridges like the .360, .375 flanged NE, 9.3x72R and so on.
These limitations only spurred American wildcatters to develop more potent deer cartridges to fit into the rules and the now popular actions, though some forgotten numbers like the .351 WSL and .401 Winchester Self Loading would fill the bill too. Btw, the .400 Legend seems to be nothing else than a modernised version of the .401 WSL.
Remember, all the British big bore Nitro Express cartridges, the .465s, .470s, .475s and so on were created because the .450s were outlawed in India and other British colonies.