What decided how the Kepler space telescope was pointed?

The prime objective of the Kepler mission was to attempt to find "Earth-like" planets using the transit technique.

To establish that you definitely have a transiting planet requires, at a minimum, that you see three regularly spaced transits.

The Kepler mission (originally) was planned for 4 years. Thus to ensure the detection of 3 transits for planets in a 1-year orbit really requires that you observe a large set of stars continuously for that period (since the transits are quite brief).

In order to do this, you need to observe a field in which neither the Sun nor the Earth get in the way during the year. This requires you to look away from the ecliptic plane.

Then, to get a large number of stars in the fixed field of view, a direction was chosen that was close, but not in the Galactic plane and viewing along a spiral arm. I believe this was done to maximise the number of stars with $V<16$ for which Kepler would supply good photometry. Closer to the plane would have given even more stars, but many would have been faint and one runs into more difficulties in terms of resolving which star is actually the variable when there is too much confusion with many sources.