Hmm… this happens waaay too often… that I write such an unnecessarily long post. Here’s basically all I said there, said in a simple way: Slavic word for “what” was originally “чьто/čĭto”, but it went through many vocal changes:

(in Eastern and Southern Slavic languges):

(Čakavian uses the short version: чь->ча (čĭ->ča))

(in Western Slavic languages, they preferred to use the genitive form of чьто: чьсо)

Also, semantic changes are very common in languages, so to mean “what”, also the word “къіи” was used (which in modern languages means “which one” or “who”): “къіи” to mean “what” in Slovenian and Kajkavian – written as “kaj”, however by time, it was kept as the only word for “what” where “чьто” was lost.

Same case happened in Bulgarian and some Serbian dialects, where “какво” (originally meaning “what kind (of)”) was used to mean “what” – and by time kept as the only word for “what”, often shortened to “кво” in dialect, and losing the “чьто”.