you generally select a tool to effectively and efficiently complete a job with the least effort, or chance of screwing up the job at hand,
example if your intent was to drive a rail road spike a tack hammer might be easy to swing and carry ,to the job in your belt loop,
but about useless for the intended purpose at hand,
and you certainly would not select a 16 lb sledge to seat tacks in leather upholstery trim
in a similar manor you would generally avoid selecting a handgun cartridge or type of pistol or revolver, for defensive use,
that was unlikely to be able to rapidly and effectively stop an attacker with a single, or at least two center mass hits,
even if its easy to carry in a vest pocket,
and you would not need something like a 44 mag or 500 AE ,
as you would very likely have projectiles pass through the attacker ,
and the wall behind him to potentially injure others, and its unlikely to be easily carried or concealed.
you logically balance options and find a reasonable compromise
thats why cartridges like a 9mm, 38 spc and 38 super, 357 mag 45 acp, 40 S&W and 10MM,
have been used for decades with good results,
they provide a decent level of power in a reasonably sized handgun.