Why? Drive down an interstate. Now drive down a gravel road that's two steps removed from being even a county road in a county with a population matching that of the Enterprise-D. That's the difference.
The principle behind transwarp conduits (which was inferred well before the hub in the series finale, like Brunali being near a major Borg road) is the same as a dirt trail through the woods- it makes movement a lot faster than trying to walk through the undergrowth and all that. That's how paths form, one animal, person etc takes it, another looking for a way through notices everything has been trodden down, then another and another and another until the ground is too compacted for stuff to grow and there you have it, a natural path. Another possibility is drag. Being in the air current behind another vehicle reduces drag. Transwarp going so fast, this drag might leave a lingering ghost on that route.
The hub tries to physically maintain transwarp conduits with those metal rings. The only way to maintain conduits without it is sending Borg ships down that conduit at various intervals.
And we've seen 3 types of transwarp
1.) Renegade Borg transwarp ("Descent")- it looks blue
2.) standard Borg transwarp (Voyager)- it looks green
3.) Voth transwarp- regular warp, only with more rainbows (must be skittle-powered)
and
0.) Warp 10, AKA trans-warp- accessed via strange dilithium found on suspect-looking asteroids. Produces the same affect as licking toads or dropping acid ("dude, I was everywhere... at once!") and turns the user into a transwarp salamander. This has been reconciled with the sawtooth graph, that the Borg found a way past the warp 10 barrier and thus avoided being cyborg salamanders.
... and there's whatever the ship in Star Trek III was supposed to have, but that came off as New Coke more than a true transwarp.
and we've also seen:
Slipstream ("Hope and fear")- bluish-gray, very advanced, very hard to use. Voyager had an easier time with the Transwarp Coil than the slipstream drive.
Enhanced Warp Drive ("Course: Oblivion")- looks like regular warp, or maybe we're supposed to imagine some amazing special effects since all the effects budget was spent on making the crew look like mercurial clay.
Dolphin fuel is not as good as it's advertised and only dead people can use
whatever the Kobali use.
Revenge is a good fuel (as it must be served cold, might it involve cold fusion?) though.
Coaxial warp drive is a different way of travel.
Spatial trajectors ("Prime Factors") &
Catapults ("The Voyager Conspiracy") have problems using or using to get very far.
