One possible reason for needing a "special snowflake reason" is that if they could have developed it independently,
they would have. Despite having an empire of thousands of worlds with billions of people, it took them 3800 years to progress from J-1 to J-2 (-9235 to - 5430). This is despite knowing that Jumps longer than J-1 were possible (because misjumps can be up to J-36)! Simply suppressing research wouldn't account for it, as J-2 capability in a J-1 universe would yield an irresistible strategic and communication advantage. The only plausible explanation is that the Vilani were working with an incorrect paradigm that explained J-1 but could not be used to develop J-2 or higher -- something as wrong about the nature of Jumpspace as the
geocentric model was about orbital mechanics.
For example, even putting something into orbit wouldn't automatically destroy a faith in an Earth-centered solar system (although it should). Sending a rocket to the point on Mars's
epicycle where it's supposed to be, would definitely do it though.
The Catch-22 the Vilani would have faced is that they couldn't disprove their flawed understanding of Jumpspace unless they developed J-2, and they couldn't develop J-2 without correctly understanding Jumpspace. They would not have known how to "send a rocket to Mars and miss" to disprove the theory -- in fact, they might have been certain they couldn't! Thus the nearly four century delay.
By contrast, Terrans may have started with the same flawed understanding of Jumpspace, but they started out with not only the awareness that they didn't fully understand it, but also were soon reverse-engineering Vilani J-2 drives that manipulated Jumpspace in ways that didn't match the explanations of the flawed "J-1 only" theory.
Perhaps the "fast track" Jumpspace theory (the one that leads to J-2) is far less obvious than the one that dead-ends at J-1. Some civilizations do come up with the "fast track" theory on their own, but most don't. The ones that don't, usually end up getting J-2 and normal Jump tech progression thereafter through reverse-engineering that reveals the "fast track" theory.