Last weeks reshuffle which had the Maps and Commerce units absorbed into the Knowledge and Ads units respectively and Pichai taking over Android brings it down to 5 focus areas:
SVP of Knowledge/Search — Alan Eustace, SVP of Identity/Social — Vic Gundotra, SVP of Advertising and Commerce — Susan Wojcicki, SVP of YouTube/Video — Salar Kamangar, SVP of Android, Chrome and Google Apps — Sundar Pichai
The key distinction between Drive and Reader is that Drive is pretty much the embodiment of Google Apps itself, whereas Reader was an orphaned product which didn't have a home in any of these 5 post-Page areas.
Drive is now a pillar of Google's services, not just Apps but Google Now and search rely on data stored in it, and with many competing storage services around any differentiating feature is worth having.