Why doesn't Chrome recognize mobile Blogger templates on the Nexus 7?
Without seeing a concrete example, I can't really tell, but it is likely it's because of the way mobile detection works is usually by screen resolution/size. Nexus 7 may have a screen large enough that the it is being detected as a normal desktop/laptop.

With certain designs, a separate tablet layout may be unnecessary if the site are usable with both mouse and touch input.