Aside from the occasional super-advanced aliens, all the major spacefaring races we've met over the years seem to be on roughly the same level of technological advancement, within say, a few thousand years of us Hewmons. Now, given how little a thousand years is on a cosmical and evolutionary scale, and how much a civilization can advance in a thousand years, you'd expect to see more races with say 30000 years of advantage (not to go into millions of years, since any civilization that old would probably look something like the Organians or Metrons and would be on a whole another plane of existence).
Instead (browsing through Memory Alpha for dates) we get:
-Klingons - only united under Kahless in our 9th century, and I suspect they didn't really attain warp-level technology until they liberated themselves from the Hurq (14th century) , taking over their technology
-Romulans, who are just an ofshoot of the Vulcans, probably from around the Vulcan Time of Awakening (4th century)
-Vulcans, probably the most ancient of the major races - the earliest we can say about their technology is that they had interstellar travel by 9th century BC (the P'Jem monastery) - not necessarilly even implying warp travel
-all the other important races (or at least their current incarnations) - Cardassians, Denobulans, Andorians, Tellarites, Ferengi seem to be tecnologically younger than the Vulcans
-now granted, it would seem Bajoran civilization is as old as 500 000 years, but again, by the 'modern' times they are no more advanced than the others; it even seems subwarp sail-ships were their best technology only a few centuries ago.
So, asside from dramatic neccessity, what gives?
A few things come to mind:
-after reaching a certain level of advancement, civilizations become stagnant, developing very slowly onwards
-highly advanced civilizations have a tendency to destroy themselves; we know exactly that happened to the Iconians and Tkons, both several hundred thousand years ago; possibly something similar happened to the super-ancient Bajorans
-some kind of 'leveling' event happened a few thousand years ago, happily coinciding with with us passing from prehistory to history, and bringing down all the other advanced civilizations down to roughly the same level with us
So, my theory goes something like this - sometime after the Iconians, another extremely advanced civilization spread through the entire known space, bringing under it's rule all the other advanced species (including possibly pre-Vulcans, pre-Andorians, etc). But a few thousand years ago it somehow collpased in ruin, pretty much dragging all the other previously advanced races back to the stone age and possibly erasing much of the memory and traces of itself and previous civilizations. Still, some of these other races, among them the Vulcans, soon managed to start their civilization again, eventually (again?) developing warp drive etc. Later they were followed by others.
What about us? Well either we were unlikely lucky that this collapse erased the advantage of most other civilizations at just the right time, or we were also under the rule of this race and were 'leveled' along with others (Atlantis?).
Instead (browsing through Memory Alpha for dates) we get:
-Klingons - only united under Kahless in our 9th century, and I suspect they didn't really attain warp-level technology until they liberated themselves from the Hurq (14th century) , taking over their technology
-Romulans, who are just an ofshoot of the Vulcans, probably from around the Vulcan Time of Awakening (4th century)
-Vulcans, probably the most ancient of the major races - the earliest we can say about their technology is that they had interstellar travel by 9th century BC (the P'Jem monastery) - not necessarilly even implying warp travel
-all the other important races (or at least their current incarnations) - Cardassians, Denobulans, Andorians, Tellarites, Ferengi seem to be tecnologically younger than the Vulcans
-now granted, it would seem Bajoran civilization is as old as 500 000 years, but again, by the 'modern' times they are no more advanced than the others; it even seems subwarp sail-ships were their best technology only a few centuries ago.
So, asside from dramatic neccessity, what gives?

A few things come to mind:
-after reaching a certain level of advancement, civilizations become stagnant, developing very slowly onwards
-highly advanced civilizations have a tendency to destroy themselves; we know exactly that happened to the Iconians and Tkons, both several hundred thousand years ago; possibly something similar happened to the super-ancient Bajorans
-some kind of 'leveling' event happened a few thousand years ago, happily coinciding with with us passing from prehistory to history, and bringing down all the other advanced civilizations down to roughly the same level with us
So, my theory goes something like this - sometime after the Iconians, another extremely advanced civilization spread through the entire known space, bringing under it's rule all the other advanced species (including possibly pre-Vulcans, pre-Andorians, etc). But a few thousand years ago it somehow collpased in ruin, pretty much dragging all the other previously advanced races back to the stone age and possibly erasing much of the memory and traces of itself and previous civilizations. Still, some of these other races, among them the Vulcans, soon managed to start their civilization again, eventually (again?) developing warp drive etc. Later they were followed by others.
What about us? Well either we were unlikely lucky that this collapse erased the advantage of most other civilizations at just the right time, or we were also under the rule of this race and were 'leveled' along with others (Atlantis?).