Why is the use of tuples in C++ not more common?

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.

Because it's not yet standard. Anything non-standard has a much higher hurdle. Pieces of Boost have become popular because programmers were clamoring for them. (hash_map leaps to mind). But while tuple is handy, it's not such an overwhelming and clear win that people bother with it.