I've been asked quite a few times in the past as to why Tamiyas are so collectable? The short answer is that I really don't know. In terms of collectability there are many factors in Tamiya's favor.
Be First - Tamiya was one of the first to introduce mass-produced reliable radio controlled vehicles back in the late 70's that actually had spare parts. This was a pretty novel idea back then. While other companies were making disposable toys, Tamiya made a product that appealed to the hobbyist. Here was a "toy" that could be repaired, customized or upgraded... amazing!
Have Quality - Tamiya was the first to introduce a reliable and high quality product that was easy to assemble. Some of the first kits from other manufacturers were horrible in comparision. Instruction manuals with no diagrams or pictures, parts that were so out of tolerance that you spent most of your time filing down stuff so that it would fit.
Then Tamiya arrived on the scene. Here was a product that had user-friendly instruction manuals that a 10 year old could read and parts that fit together so smoothly that all you needed was a screwdriver and hex wrench to put a car together. And the radio controlled car industry was changed forever.
Great Timing - Their first products were on-road cars, but Tamiya later expanded to off-road vehicles. In my opinion, it was this expansion to off-road vehicles that caught the imagination of the consumer. With off-road cars, everything was exaggerated; tire size and suspension movements. They were fascinating pieces of minaturization! For car enthusiasts, these Tamiyas were scaled-down versions of the real thing. For kids and and R/C enthusiasts, it opened up a whole new world of versatility in car. For the first time, they were able to take their cars off the streets and into the backyard, over mom's garden, through small puddles, and out to the park!