Explaining the Absence of the XindiObviously the creation of the Xindi and their inclusion in 'Enterprise' was a story-telling tool used for a variety of purposes. While their existence and the events of Enterprise's Xindi Arc do not violate Star Trek canon, they don't necessarily support it.
Fortunately, there any number of answers to questions raised by the third and fourth seasons of 'Enterprise', none of which require too great a leap of imagination to consider.
Primarily, we must ask ourselves why the Xindi seemed to possess propulsion technology that far outstripped tradional warp drive, but was never seen in "subsequent" Star Trek timelines. We must also wonder, if the Xindi were such an influential aspect of the Federation of the 26th century as stated in "Azati Prime", why did they never appear in any of the 23rd and 24th century series?
Below is an attempt to reconcile these issues.
Xindi Subspace Vortex TechnologyIn the year 2153, following a devastating attack on parts of Florida and Venezuela, Earth-Starfleet made first contact with a previously unheard of race known as the Xindi.
The Xindi had been warned by the transdimensional beings known as the Sphere Builders (or "Guardians" in Xindi society) of a supposed, impending attack by Earth. The Sphere Builders then proceeded to aid the Xindi in a preemptive attack on Earth with the ultimate goal of its destruction.
In an attempt to stop them, Starfleet dispatched the Starship Enterprise, NX-01, to the Delphic Expanse, the badland said to be the home of the Xindi.
Once there, the crew of the Enterprise was able to observe the operations of Xindi vessels, colonies, and fleets, and their technology.
One of the most striking aspects of Xindi technology was their use of subspace vortex technology, a theoretical mode of faster-than-light (FTL) transport that involved the actual penetration of subspace, an underlying dimension of normal space. While most ships were observed to only have the ability to travel a few light years, Xindi-Aquatic vessels (namely Cruiser-type ships) were witnessed using spacial vortices to traverse great distances, such as the distance from the Expanse to Earth, a trip that would normally take six weeks made in a matter of hours.
While most technologies of the five Xindi species were developed naturally through centuries of refinement and progress, the vortex transit was one that both accelerated the Xindi's evolution into a space-faring people and also marked the first time the Guardians significantly aided in the progression of the culture.
Though the Guardians didn't simply hand the technology to the Xindi, they nonetheless spoon-fed them the fundamental principles of subspace theory and let them develop the basic machinery on their own. Following nearly a century of experimentation, the Xindi finally succeeded in penetrating subspace and made their own first FTL flight at the beginning of the Earth-year 1890. Over the next three hundred years, the technology would advance, but one key component would stay the same: Subspace manipulation.
In the Expanse, though not known at the time, the Sphere Builders were at work in modifying the very fabric of space, attempting to make it habitable for their own kind to colonize. While not existing in any one realm, they were nevertheless able to manipulate the laws of physics in varying dimensions and managed to create a network of tunnels throughout subspace that spanned even beyond the barriers of the Expanse.
While the Xindi had developed the technology to enter subspace, it was not without the subspace tunnels that they were able to travel beneath normal space.
Over time however, the deception of the Sphere Builders became known and, with the help of the aforementioned Starship Enterprise, the Xindi were able to defeat the Sphere Builders and prevent them for further altering their region of space. The resulting affect of the Sphere Builders' withdrawal from the Expanse was the very collapse of the Delphic Expanse itself. The thermobaric cloud perimeter that isolated that region of space from "normal" space dissipated, the vast fields of spacial anomalies were receded and, unfortunately for the Xindi, their network of tunnels within subspace was completely destroyed.
Without further contact from the Sphere Builders or access to their subspace manipulation technology, the Xindi were forced into more than a decade of exile, during which they adapted existing technology from various other "Expanse" species to create their own version of warp drive.
In that time the Xindi species made great strides toward unification and used their new FTL propulsion methods to rally behind a program to search the Alpha and Beta Quadrants for a new planet to flourish on.
Ultimately locating a suitable system of planets deep within the Beta Quadrant, far beyond the Klingon and Romulan Empires, the Xindi spent the next three centuries in isolation, rebuilding their society and creating a unified civilization.
In the summer of 2417, the Xindi formally petitioned for United Federation of Planets membership and were accepted within the next year.
(Theory by Paul Torrez)