I presume a conservative phase of social development and a tendency toward nostalgia. Perhaps the Vilani influence, perhaps just because that is the normal way of humans(modernism is really only 500 sum years old and may wear itself out sooner or later). In any case the need for this is to justify the prevalence of customs that are implausible assuming 3000 of continuous development.
Also some of the obvious "retro-ness" from the point of view of 3000 years into the future can simply be a deliberate shout out to nostalgia for aesthetic or propagandistic(it seems whenever rulers want a real show, they do use old-fashioned imagery so it can be believed)purposes.
I like that. It has closure. It also fits in with a sort of retro-Victorian-in-space kind of imagery, fused with sleek modernistic trappings; customs and paraphanalia. So you might have a noble waering some strangely militant uniform with medals, braid and apulets, maybe a handlebar moustache, but a gauss pistol holstered on his belt, and maybe a couple of com dots.