They had advanced computing, a whole region was full of it: IX.

The rest of the empire was too feudal and too religious to dabble into advanced technology. And the other players (the BG, the Tleilalxu, the Guild) were against it as well.

What good are drones, except as weapons of assassination? Can't use them in an conventional war, there are shields. And they're very expensive, remember, the Great Houses don't have advanced manufacturing capabilities (that's IX). And, finally, there are no great wars in the empire, most houses keep to their planetary fief; the Baron's attack on the Atreides' forces on Dune was an exception. It was also mindbogglingly expensive, 80 years of spice profit.