Yeah considering the Kelvans don’t colonize, they conquer and rule!
Thanks for the info everyone, what got me wondering is on TNG characters will explicitly say what a utopia…earth is…not the federation as a whole but earth. And with how easy it is to join the federation seemingly I can imagine it takes a couple decades for living standards to rise on member worlds.
So say a relatively poor and war ravaged planet like Bajor joins, what stops an immediate wave of Bajorans selling everything they own for a ride to nicer more utopian planets of the federation? Hell it could be millions they would have to absorb fairly fast.
“Spirituality”.
Seriously, some ethereal quality (“[Our planet] defines who we are as a people.”) has been trotted out several times to explain why people on a doomed and/or inhospitable planet don’t just get the heck outta there.
One of the few occasions where an alien gladly (indeed, eagerly) traded up by abandoning her homeworld was in TNG’s “First Contact” (the fourth-season episode, not the film) which is one of my personal favourites.
With the exception of Talos IV. 
There was a TNG episode where this Irish colony had to resettle. I remember it primarily because their campfires set off the fire control in the Enterprises cargo bay. There was this very cute Irish girl that Riker flirted with. Can’t recall the episode name.
At least in this case a entire colony had left earth and settled elsewhere.
TOS had the doomed colony that the Federation thought was dead. Instead they had been sprayed by happy plants. This is the one where Spock gets happy and hangs from the trees limbs. Kirk has to fight him to reverse the plants effects.
He didn’t just flirt with her. He “washed her feet.” (shudder)
That was one of those “an entire planet of X” episodes. An entire planet of 19th century Irish people! I remember one episode in which Dr. Crusher was seduced by a ghost on an entire planet of 18th century Scottish people.
On the few occasions when we see Earth, it looks like the cities are small and compact, with vast tracts of undeveloped land surrounding them. This suggests that Earth, at least, has very strict immigration controls designed to keep it’s population in check.
I seem to recall hearing that earth has quite a large population, but I’m not sure. It would make sense to me that most people live in densely populated cities.