What decided how the Kepler space telescope was pointed?
Changing fields breaks the timeline of the observations.  If you have variation with a period similar to when you switch, you might completely miss it.  Even if the periods are different, it degrades your observations and makes it harder to draw conclusions.  The more uninterrupted viewing you get, the better.  

This mission document covers some of the reasons for the choice of the field.


Continuously viewable during the mission (you can't pick something where the earth, sun or moon constrains your viewing).
Rich in stars.  Observe as many as possible.  
The size of the sunshade limits the field to be more than 55 degrees from the ecliptic.


Although it can image lots of stars, the actual field isn't all that large.  Attempting to pick a single field with some fraction nearby would be difficult, and was not Kepler's mission.  In fact the resource of follow-up observations from ground telescopes was mentioned as one reason a northern field was chosen over a southern one.