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To introduce a causal secondary sentence, I often saw pro tio, ke and ĉar.
Recently, I saw pro ke, without any tio to which I think ke is supposed to refer to.
Mi ne povas dormi pro ke tro malvarmas kaj la hundo bojadas
This surprised me because it is uncommon. However, "por ke" is common, so if "por ke" is valid, is there any reason for "pro ke" to not be valid?
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:
The meaning of por ke and por tio ke are different.
As illustrated by the following sample sentences.
The sentence you list with pro ke seems awkward to me and would read better with pro tio ke.
These tio-less constructions are criticized in PMEG (§33.2.3) but no objection is given other than the fact that they are not traditional:
Nur por ke, malgraŭ ke, anstataŭ ke, krom ke kaj sen ke estas ĝenerale akceptitaj. Sed nur malkutimeco estas argumento kontraŭ frazoj kiel: Ni parolis, pri ke la prezoj ĉiam altiĝas. Oni argumentis, kontraŭ ke ili ekloĝu en la urbo. Li sukcesis eskapi, pro ke vi dormis. Tio okazis, antaŭ ke mi naskiĝis. Malkutimeco estas tamen forta argumento. Oni ne senbezone esprimu sin en maniero tiel nekutima, ke oni eble ne estas komprenata.
In PAG (§124, §267) the authors say merely that per ke, pri ke are "ne uzataj" and that pro ke is "nekutima ĝis nun."
It is likely that some readers will see these expressions as "bad style"; however (speaking for myself) the cited sentences seem clear as day to me, and there are no good grounds for calling them ungrammatical.
Zamenhof was quite happy with por ke, malgraŭ ke, etc, and the absence of pro ke in early texts is easy to understand given the neat alternatives: ĉar; tial ke.
Not accepting them effectively introduces an irregularity into Esperanto, and a new unnecessary rule.
The insertion of tio can be slightly confusing, as the reader or hearer naturally assumes that tio refers to something already mentioned, and instead must do a mental double-take: Je la kvara horo la pluvo komenciĝis, kaj mi revenis pro tio, ke mi ege malsatis. Or: Mia amiko alvenis malfrue, kaj kiam li sidiĝis, mi longe parolis pri tio, ke nia enspezo senĉese malkreskas. In English we have to insert the fact that or the idea that or resort to cumbersome periphrases: pri la fakto ke...
