This is a question about paid advertising, not cross-site promotion or community ads.
The Stack Exchange Media Kit has a list of sites that accept advertising: besides the SO, SU, SF, they are Android, Arqade, Ask Different, Ask Ubuntu, DBA, Drupal, Game Development, Programmers, Information Security, SharePoint, Unix/Linux, Web Applications, and WordPress.
By what criteria where these sites chosen? It can't be just their traffic: several sites have higher traffic than those listed above, yet have no ads (ELU, Mathematics, Seasoned Advice, Home Improvement, TeX, Electrical Engineering, Graphic Design, Travel, ...).
Has it been decided to limit advertising to technology sites, with all other categories being free riders? But even then: EE, GIS, and Salesforce are within the technology group, with traffic greater than Drupal's, yet with no ads.