Domestic intelligence obtains evidence legally to use in court although may use illegal means at times to know what evidence to collect. It's much closer to a police force. Foreign intelligence only cares about not getting caught; everything it does is illegal. Comingling domestic and foreign intellifence could produce complications because the illegal foreign activity could be exposed in court.

There's two main branches of domestic intelligence, pure domestic intelligence, such as against domestic terrorist groups, and counter-espionage, which fights foreign intelligence agencies. Those two skillsets by themselves are very different from foreign intelligence. It's like asking why a quarterback doesn't play safety.