There is apparently a genetic limitation factor for DUST implants, similar to the Capsuleer one. It is a rare few who are compatible with the DUST technology, thus artificially limiting the proliferation. This also installed another way to make certain that you can't be both - the genetic viability for one invalidates the other.
This is the excuse that was sold to me, I am not certain if it is canon or not. I raised the same topic of "you'd have to be insane to let this technology in the wild" when the idea of DUST troopers first came up.
I also heard this, but I've seen no source for it, so my current feeling (as evinced by the fact that I've ignored it) is that it's purest speculation. Even if that weren't the case, the solution is one clonejack away,
and clonejacking is PF.
The "genetic lottery" theory for capsule viability seems to me to be related almost entirely to being able to tolerate capsule training-- it's suggested that the technology's alien sensory input induces severe nausea in most, and then there's the danger of mind-lock or wetgraving, both of which I presume are, again, products of the capsuleer's mind-taxing cybernetic interface.
Cloning tech, on the other hand, shows no sign of being a genetic problem. If there's any kind of "genetic lottery" to be won, it's related to dropsuit control-- but I defy you to find me PF on that. The dropsuit is an anthropomorphic device on a human scale with human movements and abilities; there's no reason to think that the interface would be troublesome for the mind to process in any significant way.
The best reason for not being able to be a capsuleer and a DUST soldier
at the same time is simply that whatever cybernetic rigs are required are mutually exclusive. You can't have both a capsuleer rig and a DUST rig installed at the same time. Even if you could theoretically switch back and forth ...
(1) Any such switch would probably require highly-invasive, delicate surgery. Most capsuleer implants go around the periphery of the brainpan, but both the capsuleer neural shunt and the DUST insta-clone device appear to be kind of in the middle of things.
(2) Capsuleer stat implants and hardwire setups are non-transferable (and even if they could be transferred, they wouldn't survive the first death).
(3)
All DUST skills are hardwired. "Learning" them means increased integration with the inhabiting consciousness. These are apparently installed as modifications to the clone during consciousness transfer, and are thus most likely outgrowths of the primary DUST implant.
(4) DUST soldiers presently seem to inhabit what seem to be genetically-uniform (or genetically irrelevant) stock clones. Those that have their own "faces" have had a clone modified for casual use. This, also, militates against there being a "genetic lottery," since the only question would be whether the original DUST implant could be installed. "Oh, no, his immune system is rejecting the implant. Oh, well-- just go ahead and stop the heart now, let him wake up in his new clone. He'll get over it."
Has the stock clone won the genetic lottery?
All of this makes the notion of switching back and forth an issue of practicality, not literal impossibility. A capsuleer would have to fully retire (get all capsuleer implants removed, up to and including the pilot rig), then somehow arrange to get the DUST implant implanted.
I think I did an okay job of explaining that with Aria.Now, why aren't we seeing people all over creation getting DUST implants installed?
Well, first of all, the clones are subjectively cheap but objectively expensive. Entire planetary districts need to be controlled in order to produce them, a few dozen at a time. Equipping the citizenry with them (or replacing military regulars) would take a lot of time and a lot of money.
Secondly, there's a good deal of indication that long-term usage leads to serious psychological problems, which you don't want your whole security service suffering from. If it's not PTSD, it's recklessness disregard for one's own safety, and possibly that of others.
Third, there's the finicky nature of getting the implant installed to begin with. All indications are, it's a bit of a tricky procedure-- the implant goes kind of right in the middle of the brain.
That said, we'll probably soon see the rich and powerful getting technological immortality installed in their heads. The next person to assassinate a Ray of Matar may be surprised to find that shooting for the head is not as effective as it once was.
Edit:
Also, the "genetic lottery" thing never made much sense on its face. Otherwise, the Caldari tube-child program combined with some successful DNA samples would be a license to print capsuleers. I'd cast it more as a combination of environmental and genetic factors resulting in a certain particular sort of mental toughness or adaptability-- not willpower, necessarily; just a strong ability to accept and adjust to coherent yet highly alien sensory input and modes of action.