I can think of some reasons they didn't bother to change the tail geometry. A slight change in tail geometry would likely result in a complete redesign of the tail and maybe even the whole aerodynamics of the plane. It would certainly take some time to develop, and China has a strong need to maintain a sizable fleet of modern bombers to deal with regional threats. H-20 hasn't come out yet, so the only modern bombers currently in service are H-6 J/K/N variants. Halting the production of those variants just to change the tail is certainly not an optimal choice. Another reason that I could think of is the former tail gunner position might be replaced by some electronic equipment, hence despite the gunner is no longer present, there's still a need to keep that position, and hence they didn't bother changing the tail geometry.it is interesting the Chinese never dealt with the geometry of the tail, which was dictated by the 1940s era need of a manned tail gun turret. The turret and gunner position is long gone, but its outline and the fuselage geometry to facilitate its presemce is still there. Paint the windows back on and you wouldn’t notice the tail gun turret is gone.
It may look like an old plane from the outside, but trust me the inside has completely changed, the engines, the radar, and the electronic equipment are all brand new.