My last job was as a compiler guy at a large Electronic Design Automation company. The product I worked on was written mostly in C++, but we weren't allowed to use the STL. The reason was that the early versions of the product were written before the STL was stable across all platforms used at the company* (which included things like IBM AIX, HP-PA, etc.). So the original developers (back in the late '90s) wrote a number of container class templates that were used instead. Of course, once those were in place, you had to keep using them, as mixing STL and non-STL containers would have been a maintenance nightmare.

I would suspect there's a lot of old C++ software that falls into this category. Especially if cross-platform compatibility was/is important.

(*Actually, it was often insinuated that the STL is _still_ not 100% compatible between Linux and these more exotic platforms. I have no idea if this is true.)