Can I get some rationale from the Optimzer why it chose a certain explain plan?
Yes, you can.

This is called tracing the Oracle optimizer. Doing so creates a trace file in which the optimizer dumps all reasoning done when composing a plan.

An example of generating a trace for a specific SQL can be found here how to trace optimizer for specific SQL system wide - 10053 trace event

You might need a little time to read it, given the amount of data found in the trace, it is amazing that the Oracle even finds time to do this and return in a decent time.