Because it's not an extension to the C++ Standard Library (nor to the STL, naturally).
It is a third-party distribution, that you must download and install, locally and (for some Boost libraries, if you dynamically link) on the target system. You must manage and document the dependency.
I shan't enumerate all the scenarios in which this is not feasible, but it should be self-evident that you cannot always use non-standard code. Not everybody is working on a platform on which you can simply write yum install boost-devel
, write your code and move on. The world of computers goes far beyond commodity desktop PCs.
That being said, most arguments for avoiding Boost are incredibly weak, due to its extreme portability, and the fact that the majority of Boost libraries are header-only (which reduces the packaging overhead significantly).
Seems like a lot of fuzz for nothing
I don't think writing the phrase "if you can use Boost" can be honestly described as "a lot of [fuss]".