In the second case it's their own ignorance and lack of knowledge. Since the dalish live pretty well. Not perfectly but better than the city elves
Debatable,
***Maybe Minor Spoiler, but not really a spoiler, don't read if you don't want to hear a single word related in anyway to Masked Empire***
in Masked Empire Gaspard states that he attempted several times to gather a small army to hunt Dalish, but was blocked by Celene. What this means is that under every other emperor/empress hunting Dalish clans for no particular reason other than they were there is fairly common. CE at least have some rights (very limited and circumstantial I'll admit, but nobles don't run through Alienages killing willy-nilly as that leads to costly and short lived rebellions), whereas Dalish can be attacked by nations who don't want them (everyone except Rivain and maybe Ferelden) and Templars (who quite thoroughly dislike mages outside their control) and bandits (although I'd guess that bandits don't fare so well against trained elves).
Also, if you care to look at real world examples of nomadic peoples (hungarian tribes, Romani, Early Slavic cultures) they rarely faired well once so-called 'civilization' made its way to them. If you think about it, the Orlesians dislike having a few hundred (maybe a very few thousand) Dalish wandering through unused territory in the periphery of their empire. Imagine if thousands (even tens-of-thousands) of City Elves left the cities and joined the Dalish? Having that many people wandering unchecked through their borders and oweing no loyalty to Orlais would be a serious cause for concern. If the CE did migrate out of cities I'm absolutely certain that the Orlesians would march on them and force them back to the cities (while making numerous examples of them).
One last point - Living off the land, as the Dalish do, without agriculture has a very high cost on the land. There is a reason why there were so few native Americans in America (per square mile) as compared to the population of New England just prior to the revolutionary war. The land can only support so many people, agriculture can support a whole lot more (even basic agriculture like Thedas would have), so having more than a handful of CE leave would be impractical and would simply result in hundreds and potentially thousands of death due to starvation.