It seems to be unique for every species. The Voth were certainly more advanced (more advanced than the Borg, even), and had been around for millions of years, but didn't seem on the cusp of becoming energy beings. Assuming the Dyson sphere builders were corporeal (why else would they have built the sphere?) they probably also were thousands of years ahead, technologically. The Zalkonians on the other hand seemed roughly as advanced as the Federation (with the exception of their breathing-stifling weapon), and they were on the edge of that transition. Perhaps other factors come into play as well (e.g. the attitude of said species, though the Zalkonians didn't seem that open minded).
The Zalkonians definitely put a question mark on whether a species advances evolutionarily once they hit a certain technological point.
I honestly don't think technology is the factor, though certain technologies probably help the species get to a point where, evolutionarily, they evolve into energy beings.
As you said, the Voth were certainly among the advanced races in the franchise that we saw, and they didn't seem to be on the cusp of any next level of evolving.
I think a bigger question is this: why did so many of the highly advanced species out there die out hundreds of thousands of years previously?
The Iconians were bombed out, but they couldn't have all been on just their homeworld. They had to have colonies.
Same with the T'Kon Empire... a single star going nova couldn't have wiped out their entire empire. (Though what happened to Romulus sort of gives credence to this.)
The people from
"The Chase"... we have no idea what happened to them. Though I have wondered if they evolved into Changelings.
The Kalandans from "THAT WHICH SURVIVES" were clearly extremely advanced, though McCoy theorizes thecsame disease that killed the peoole on that outpost killed the rest of them.
Who built the Guardian of Forever?
Who built the Dyson Sphere?
How about the Preservers? Who were they and what happened to them?
I know I probably strayed too far from the topic, but once I was trying to respond to this post, I just couldn't stop the theorizing.
Anyway, my take on Starfleet not adopting other forms of travel...
As has been mentioned, they seem to play it ultra safe. If even a hint of failure shows, it gets shelved.
Lack of resources might also be a huge factor, such as the rarity of what was used in "TIMELESS".