The developement of PIE sonants

This is my first message on Cybalist. I,m not a professionalist and I have been interested in IE languages (and also in Uralic ones) for a little time, just for satisfaction and improving my knowledge...

I have a question about the development of PIE sonants (*r, l, m, n) into corresponding sounds of the Proto-Slavic Language (personally, I speak Polish as my mother-tongue).

Why some of the sonants developed into PS "soft"(palatal) sonants while the other developed into "hard" (non-palatal) ones? I can give an example: PIE wlkw became PS *våk (so there is a palatal lateral sonant). However, PIE *bhorko developed into PS *brk with a "hard" sonant.

I'd like to add that the PS syllabic sonants are often reconstructed as a consonant followed or preceded by a yer (a very short vowel u or i).