The ideal primary knife is a small knife located where you can easily reach it with both hands. For most divers who wear backplate and harness systems, this position is most accessible on the waist belt of the harness. Having the knife off the leg helps divers avoid entanglement with a guideline which is most often present during technical dives in caves or wrecks. Because cave divers are always in close proximity to a guideline and because there is little need or use for larger knives in caves, a small knife is carried, and positioned where a poorly placed kick will not trap or break a guideline. Such streamlining is also very relevant to wreck diving and geberal open water diving, but protecting guidelines from knives isn't as critical for the average recreational diver because the average recreational diver rarely uses a reel or guideline.
Having the knife off the leg may be of benefit to those who dive in water where monofilament line and other entanglement hazards exist. For those divers who penetrate wrecks, they will often need to carry two, three or more cutting tools to deal with possible entanglements and multiple entanglements during a single dive from everything from fishing line to wire. Wreck divers use reels and lines as much as cave divers and there are more points inside and around wrecks to entangle a diver. Equipment streamlining and reachability is important to minimize dangers associated with diving around wrecks and where fishing boats tend to prowl.
The traditional diving knife is a large knife and despite the size is still a tool. Used for everything from a ruler for measuring game when hunting, to a screwdriver, to a cutting tool, to a signaling device, to a hammer, the traditional diver's knife is big and bulky. It is often most easily and best worn on the inside of the calf where it can be reached with both hands, but not as easily as a small knife on the belt, and where its position will help minimize entanglement by keeping the outer part of the calf smooth to allow fishing line and other lines to pass if a diver brushes up against them due to lack of awareness.
Some divers will find intelligent ways of mounting these large knives elsewhere, where they are able to easily reach them with both hands, where they can prevent the risk of accidentally cutting buoyancy or life support equipment when deploying and restowing them, where they are comfortable and where they will minimize entanglement potentials, but this all depends on the specifics of the type of scuba and BCD system a diver uses.
Keep these factors in mind and you can best determine how and where to carry your knife safely and comfortably.