What decided how the Kepler space telescope was pointed? I suppose that the Kepler field was well chosen. It is rich in stars and has minimal background light since it is pointing out from the disc.

But other things could have been considered. For example, a field which contains the most stars which are near enough to be followed up by ground based telescopes. And why wasn't Kepler designed to change its pointing so it could cover several fields, such as all of the above? Funny enough, it does so now as K2 after two of its reaction wheels have broken, so studying only one field doesn't seem to be a technical issue, but something which astronomers prefer. Why?
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.