My reasoning is simplicity, and cost. I currently buy 22lr, 9mm, 5.56, .308, 12 and 20 gauge. Beyond the "right gun for the job" angle there is the overall saturation of those cartridges, generally meaning cheaper prices if bought at the right time. To get into the more tin-foil hat aspect, if SHTF, those ammo types would be more likely found or scavenged, or shared between family and friends.
Though like you mentioned, having a target gun, and CCW gun, or having tactical rifle in .223, and also a varmint rifle in .223, and so on.