Why are some conferences no longer indexed by 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.