This is a great question. It deserves a great answer. This is the best I can come up with on short notice.
You're absolutely right that por ke is common and pro ke is basically unheard of. My advice: do not use pro ke.
Why is this? My first thought was "tradition" (i.e. "just cos"), but that's not always a good answer. My second thought was that traditionally we give more flexibility to por to interact with more parts of speech than we do for most other prepositions. (Por is frequently used with infinitives, for example, while most other prepositions stick to nouns.)
My impression, though, is that there is a good semantical reason for saying por ke and avoiding pro ke. Consider the alternatives:
- pro tio ke (common)
- por tio ke (not common)
The meaning of por ke and por tio ke are different.
- por ke = in order that, for the purpose of
- por tio ke = for the following reason
As illustrated by the following sample sentences.
- Utilis por tio ke multaj ministroj de la nuna registaro estas dungitoj de okcidentaj entreprenoj aktivaj en Kongo.
- Faru ion al la homoj, por ke ili nenion al vi faru
The sentence you list with pro ke seems awkward to me and would read better with pro tio ke.