Reddit reviews Microstrip Lines and Slotlines, Third Edition (Artech House Microwave Library (Hardcover))

There are limits to how fine a feature you can make, and it takes more expensive processes to make finer features. With a lower Er, feature sizes can be larger, therefore you can make circuit elements with tighter tolerances.

Also for antennas like patch antennas, physically larger antennas are more efficient (don't let the substrate get thicker than 0.1*wavelength or surface waves become a problem).

I can recommend "Microstrip Lines and Slotlines":

http://www.amazon.com/Microstrip-Slotlines-Artech-Microwave-Library/dp/1608075354/

Older editions are okay, too. I like the books that Inder Bahl writes.