why would you need technology if you had magic? Also half of Thedas would be in Qunari hands.
As TKS says, because the Qunari already proved that their technology can match the Magisters' power. And technology has more room for improvements that can be applied quickly and en masse.
But that is the problem. Magic is a scarce resource in Thedas and its control depends on the power structure of each society. A magocracy wouldn't be interested in technology not only because of apathy, but because of fear too. It's interesting to note that so far the most interested in Qunari technology have been dwarves (Dworkin and Javaris).
However, if a magocracy would put aside its fears of social revolution and embrace technological progress, they could become one of the most dangerous forces in Thedas. The change would be traumatic, though.