Frankly, it's hard to understand why you'd want to go.

Literal

Frankly speaking why you [subject-が] want-to-go [nominalizer-のか] hard-to-understand.

率直にいえば uses hiragana いえば instead of 言えば — same meaning. 理解しにくい uses ~にくい (hard to do). なぜ (why) opens the embedded question.