Many people say ALL Slavs should be Orthodox, why is that ?

Probably because majority of Slavs are, but religion=/ethnicity or meta-ethnicity in this case.
Concerning your question, yes quite a few members here are "practicising" Eastern Orthodoxy. Also, yes there is inculturisation of some practices of folk religion in folklore way we celebrate our holydays, but it does not Slavic Orhodoxy different religion from Greek or Romanian.