1. Cost vs Reward.
2. Regulations.
1) Many concept cars are non-functional or only partially functional. Making them fully functional and production ready, even at prototype levels would be extremely costly.
2) Many features in concept cars are not evaluated against FMVSS (Federal Motor Vehicle Safety Standards) and other standards, they are not crash tested, they may not meet EPA regulations.
There are a lot of other reasons, but they all basically boil down to cost vs reward.
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There is also the backwards case, where many modern "concept cars" are actually just pre-production teasers with fancy cosmetics.
Disclaimer: I work for GM, but not on concept cars, sadly =(