In some cases, failure is a simple one: they do not really scan the app code. AV-Comparatives found that only use whitelist apps or blacklists, and sometimes they are very broad. They can allow all apps that the package files start with "com.instagram," but it's worthless to create the heart of apps that use others in that name.
Pass apps come from familiar security brands, with big names like AVG, Kaspersky, McAfee and Symantec all acquired. However, those who have fallen short are a familiar pattern. Many of them are written by amateurs, cookie-cutter apps or from companies that are clearly not security-oriented. Anti-malware apps from 32 test vendors have been lost for two months since the trial took place in January.
It's safe to say that it serves as a reminder to stay on antivirus tools from companies with solid track record. However, it also illustrates the challenge facing Google and other store operators in screening apps. They can verify that apps do not cause harm to users or violate the law, but they can not enforce the baseline quality level required to keep your phone safe.