I remember Google explaining it back when the Galaxy Nexus was released.
Found it:
http://www.androidcentral.com/why-ne...ave-no-sd-card
We got tired of seeing OEMs include many GB of internal storage for music, while users were still running out of space for apps and data. This approach lets us merge everything on one volume, which is way better.
-- Dan Morrill, Android engineer at Google
The lack of a SD card is the only issue I have with my Nexus 4. The 16GB version has about 12.5 gigs of usable space, which gets eaten up fast when I have music, a nandroid backup, Titanium Backup of my apps plus the apps themselves. I wish They had a 32 GB option like the Galaxy Nexus had, that was plenty for me.