Well it only applies to a society that has not met consistent psionics yet. If another species uses it then attempts to copy might be met with complete failure despite doing it exactly the same.DA example is a great one. Imagine a man who goes out and throw a fireball. It's actual fireball, it's plasma ball created from nowhere, flying to a target and blowing. Because this man is weaking the veil by their very presence.
Of course, I can imagine materialist who denies "all that crap" about spirits, transcedence and so on. But will he deny a fireball?
If met with concrete proof, like fireballs or a clairvoyant who can consistently guess what people are thinking in the next room or something, then a materialist would stop claiming it's bogus. That's not the same as being able to replicate it, which gaining the techs would imply. For all we know the "spirits" of the Shroud might be withholding access to entire species when they've made a choice like large scale cybernetic implants, or perhaps it's incompatible with the Shroud itself.
The restriction is similar to the choice we need to make about FTL types. Once you lock on to one way of thinking you have no reason to embrace the others, it at least makes sense why you don't develop them yourself.
That doesn't explain why you can't shift after witnessing how other people do it, but then it's the same problem. I can think of a reason why the choices can't be combined, but not why you can't change your mind if you were willing to give up the other way of doing things.