Reinstating the Hatfield connection would be very problematic. Firstly the route is severely severed at the Hatfield end by the building of the A1 road tunnel and other developments beyond the tunnel and up to Hatfield station, and at least one new estate across the route at St Albans. Secondly the "Alban Way" pedestrian and cycling route along the old trackbed has become very popular as a traffic-free route and any change to it would be, I suspect, be strongly resisted by many. Thirdly, the route would require a reversal at St Albans Abbey station each time - or of course drop the Abbey station and reinstate London Road station with a significantly shorter walking route to the City station......
You can add to that the problems it would cause at Hatfield station - the junction was to the north of the station and faced south. As other posters have pointed out the ECML at the south end is close to capacity - the slows have already 6 tph just with the 4 x inners and 2 x outers.