With current technology, why is there still increased separation between aircraft and delays in low visibility conditions?
In good weather, airports can move lots of aircraft because the ATC separation standards are much less than the bad weather, poor visibility ATC separation standards. 

“Current technology” really hasn’t changed much over the years as ground based ILS CAT II and CAT III are still the most accurate ways to get get aircraft on the ground in bad weather. 

Most of the increased separation is due to the requirements to have uninterrupted ILS ground signals reaching the aircraft on approach. Aircraft waiting to take off must wait behind hold lines farther from the runway, and aircraft on final must be spaced farther apart so as to not interfere with the ILS signals. 

Once on the ground, aircraft will often experience delays getting to the gate due to the difficulty of taxiing in near zero visibility. Even parking at the gate can often require special standards and procedures.