There may be several reasons:
- Nobody has entered the information yet. The actions necessary to enter bibliographies, check for possible errors, and keep the data consistent is a time-consuming work, and our resources are very limited. Hence, it is inevitable to concentrate on a representative subset of all possible venues. Unfortunately, this also means that some publication series have to be neglected.
- We have no access to the publication venue or a reliable source of bibliographic meta-data for this venue that we are allowed to use.
- The journal/conference is a publication venue of disputable scientific quality and our advisory board has spoken out against indexing this venue. Fortunately, this is very rarely the case.
If we are missing an important publication venue, you can always try to convince us that we should include a certain publication venue.