The concept and design of Exoskeletal vehicles is nothing new. The Old Republic experimented with different designs over the past 40 to 50 years. They worked from the concept that a new sort of "urban tank" could overcome obstacles too coarse or dense for a tracked or grav vehicle.
An offshoot of this research, the MAX armor, is used today across the galaxy. Of course, dedicated teams of engineers continued to concentrate on enlarging the exoskeleton to support larger armor and heavier ordnance.
Thus, the BattleFrame came into existence. Able to carry tank-sized weaponry and supporting a defensive force shield system, the BattleFrame, on paper, was greater than any tank in the arsenal.
There were interface issues though that prevented this crowning achievement from being fully successful. Pilots trained for years to master the complexities of moving fingers, feet, knees, elbows, and all the weaponry and systems of the BFR. It's so much to keep track of that pilot responses were slowed, making the BFR less effective. Too many instances of pilot death and vehicle loss were the results of initial training and testing. The first BFR plans were then discarded and a new team took over the design based off new concepts.
Around this same time as the BFR underwent its first round of redesign, the war on Auraxis started. Republican leaders watched avidly as this savage battle erupted continuously on that alien planet. Eventually, they considered using Auraxis as an experimental test field.
Once the Republican leadership determined that the expeditionary force on Auraxis would have to be terminated, they decided to hold off for a year or more and use Auraxis as a closed test-bed for ideas and technologies too dangerous for general use within the Republic itself.
The Republic transmitted designs for vehicles and constructs through the micro-wormhole to the empires fighting each other on Auraxis. They expected nothing in return; they just wanted to find out what the factions would do with these new designs.
Only one BFR design was sent to each of the three empire.
Timeline: Two months Pre-Bending
The BFRs were an improvement over the main battle tanks of each empire, that is, if the initial design issues could be resolved. Auraxis engineers saw no use in encoding the BFR into the warpgate shunts while design problems were still evident and unresolved. The potential of the BFR was too great though. Research began immediately to fix the design and get it into battle.
The solution to fixing the BFR design came from an entirely unrelated field of research, as such solutions often do. Continued research into developing portable respawn units eventually created a "resonance crystal" based on the crystals discovered and studied within the caves.
Scientists working on the crystal tried to create a way of recording the patterns (normally stored in the warpgate shunts) within a data storage unit so that information could be saved independently of the Auraxian Core.
The resonance crystal was a close, but not exact solution. The study of Auraxian cavern tech showed that the Ancients seemed to have dealt with probability quite a lot in conjunction with dimensional research. The belief currently is that there are many potential probabilities to any event and that decisions made on the moment cause many of those probabilities to fade away while strengthening the one chosen, thus crystallizing the universe around that decision.
Extrapolating on this theory, they began creating a "possibility recorder", which eventually became known as the resonance crystal. When the crystal remains close to a person for a long period of time, and if that person is put under stress for a great deal of that time, and that stress occurs in the caves of Auraxis, then the crystal responds naturally to store "echoes" of events that might have happened to that person. (The nearly-happening possibilities that did NOT happen to the person are recorded in the crystal.)
Eventually, a kind of composite recording of that person occurs in the crystal. In fact, the crystal eventually develops enough potential on its own that it can be queried (through instrumentation reading pattern feedback) and it responds in a fashion remarkably similar to the person that it "echoes". It isn't sentient per se, more like a remarkably efficient database of knowledge with some emotional feedback based on the original source personality.
When the BFR design teams and engineers heard about this remarkable advancement, they began wondering if the resonance crystal could be used as the elusive interface element, finding a way for it to deal with the moment-to-moment details of running the BFR, while also becoming an almost perfect interface with the mind of the pilot.
The end-result was a hodgepodge of computer interface, resonance crystal "echo personality" and a bit of ancient-tech elements besides...but it worked.
Most soldiers are now outfitted with a resonance crystal implant at the top of their spine. When soldiers are sent into the caves for combat, they automatically begin to accumulate "echo potentialities". Eventually, their resonance crystal becomes properly "attuned" and they become capable of being a BFR pilot.
If the soldier pursues that field, then a socket is surgically grafted in the back of his or her neck, allowing the BFR harness to integrate through that socket, directly interfacing with the resonance crystal in the spine.
When the circuit is complete, the soldier is able to concentrate on the surrounding battlefield, only needing to tap an occasional control here and there to nudge the "echo personality" into action. That semi-sentient construct then obsessively handles the details of the BFR. This system works well, allowing the pilot to concentrate on survival while the "echo" keeps the machine in line. There are some issues. Pilots often report getting flashes of anger or empathy from their BFR, especially when encountering close friends or enemies. One pilot was even exonerated from murder after a court finally decided that the BFR acted independently when killing the pilot's recent ex-girlfriend. But those instances are rare.
However, it's becoming quite common for BFR pilots to "hear" the emotions of the echo, even when not connected to the BFR itself. There have been reports of pilots that become morose and suicidal if their resonance crystal is removed. Despite attempts to make them feel the crystal is just a tool, most pilots eventually think of it as their closest friend. After all, who agrees with you more than...you?