I lived and breathed rockets for much of my life. I launched by first Estes rocket in 1962 and later spent 28 years in solid rocket propulsion with a major company. I worked in a variety of applications from large boosters, satellite propulsion, ICBMs to small tactical motors and was involved in the design, manufacture and launching of a variety of vehicles. Over the years I amassed quite a stash of model kits, but most remained unbuilt. Maybe it was because I got to play with the real thing, but I think there were other reasons.
1.) (real) Space vehicles are not personalized. There is little opportunity, at first glance, to associate space vehicles with real people. I know very few modelers, today, who build an Me-109, they build Gunther Rall's black 13 Bf-109G2 of 8/JG 52. Few people can tell you what a Lotus 38-Ford was, but most racing buffs can identify Jim Clark's British racing green and yellow #82 as the 1965 Indy 500 winner. There is a mini industry in figuring out Michael Witmann's tank serial numbers. To build the USS Enterprise, you need to know the operational date, camouflage pattern air wings carried, etc.
The limited number of space vehicles and the fact that they pretty much all looked the same makes the 'return business' and research aspects of space models self limiting. (We need nose art on the shuttle...) As a direct participant, I can tell you the difference between the pre- and post 1986 Titan 34Ds, but the detail level on available models wouldn't show it.
2.) Compared with the options available in the passive video and interactive computer media, (real) space models don't offer as much 'participant association'. Many contributors to this thread have mentioned the fascination with computer games, movies and TV shows, but another major player is the lack of opportunity for 'imaginative interaction'.
With other model subjects, much of the 'genius' associated with the operation of the subjects, and the research on the operation is tied to the occupants... the fighter tactics used by the individual pilots, the development of individual tank battles, tradeoffs between different fighting styles, the wheel to wheel duels between famous drivers. With space subjects, most of the 'genius' is hidden in the design process. From the observer's perspective, they all make a lot of noise and smoke and then disappear in a straight line. The spacecraft occupants are pretty much pasengers for most of the flight.
Also, the technologies involved in the devleopment of cars, tanks and aircraft are visible to a large extent in the models. Technological evolution in spacecraft is virtually invisible. The currenlty proposed mars rocket (which I doubt will ever fly) looks like something made by a 12 year old out of his spares box, but bears little resemblance, technically, to the original 'derivative' vehicles. Materials and electronic advances are invisible.
3.) Of course another 'imagination' and emotional association issue is the fantasy of actual participation in the aerospace industry. The NASA of Von Braun and the WWII/cold war generation was a dynamic and inspiring organization. NASA was about the future and improvements for mankind. The 'politicization' of NASA over the last 30 years has resulted in an organization that is less than awe inspiring. It is now lead by politcally appointed bean counters and lawyers and staffed using criteria similar to the Departments of Motor Vehicles ('nuff said). Not too attractive to my imagination.