Why are some conferences no longer indexed by Scopus? I have seen in Scopus that there are some conferences that are still running nowadays, but they not longer appear indexed on Scopus. For example, in the field of Computer Science I see that the ICCSIT conference is indexed only until 2010, but now they are in the 2015 edition. I know that some indexing units such as DBLP exclude from their current rankings some conferences when their quality has decreased substantially. Does this situation also happen with Scopus?
Scopus, just like DBLP, has standards.  In fact, if you look at Scopus' description of the conference coverage, it appears that they actually just scrape DBLP to get their computer science conference listings.  As such, while I don't know about the particular conference that you mention, I would expect that anything that DBLP drops, Scopus will drop as well.