In general, they want higher salaries (like you mentioned), but there also seems to be a clear disdain for most of the portuguese cultural heritage, to be honest. This might be an ecochamber (I'd put money on it being the case) but most people "around me" who are in "the tech world" are pretty much diametrically opposed to most portuguese customs and culture. They don't really identify with the country, the history, the religion, the folklore, the politics, the sports...and if you add the income disparity to it, it makes sense that they want to leave.
If not for the will of my Significant Others throughout the years, I would have definitely left a looooong time ago.
(Man, I find it somewhat atrocious how much we like to tell the great stories of how we went and stole, raped, killed and enslaved during the time of the discoveries, all for our own pleasure (and that of our religion). I guess I can somewhat frame it in its time, but you will never hear me talk about our "great accomplishments" in that regard. I'm deeply ashamed of them, not proud.)