A cynical answer is that many people program in C++, but do not understand and/or use the higher level functionality. Sometimes it is because they are not allowed, but many simply do not try (or even understand).
As a non-boost example: how many folks use functionality found in <algorithm>?
In other words, many C++ programmers are simply C programmers using C++ compilers, and perhaps std::vector and std::list. That is one reason why the use of boost::tuple is not more common.