Uhm Then why are their so few Elven left in Thedas.
Because they destroyed themselves.
Abelas said so.
This is why there are wandering Dalish clans. The humans were not to be blamed for the calamity that destroyed Arlathan.
And byt the time they had settled in the Dales, long after they destroyed their own civilization, how could they resist a human invasion? Or any other people's invasion? They were scattered, without leadership, without military support. And right next door there was an empire slowly in expansion.
The humans were already asserting themselves as a empire in Tevinter. And the Old Gods, for some obscure reason, abandoned the elves and started whispering their magical secrets to the humans instead.
The result was further degradation of their race. In real world Hystory, that's what happens as well. Many great civilizations crumbled and the lasting survivors were enslaved by a new power, that rose and became an empire. Such was the fate of the Greeks, who had been enslaving other peoples long before the Romans.
Who knows if the elves hadn't been enslavers themselves in the glorious days of Arlathan? They may not be the fair-minded, pure hearted people we take them for...