In a related but different question to Indo-European prepositions: whence did they come?, why do just about all modern Indo-European languages have prepositions rather than postpositions?
PIE is believed to have been an SOV language, and like textbook examples of SOV languages it had relational postpositions rather than prepositions. Yet every modern Indo-European language I know of uses prepositions primarily (one or two have postpositions as well, but in such languages prepositions are a clear majority). Even Latin, which looks to have a nominal word order of SOV (though the inflectional case system allowed for large variation in word order) has exclusively prepositions (excluding the comitative pseudo-case where previously postpositional "cum" - which, I may add, is a preposition in Latin - was reduced to a suffix).
How is it that just about every Indo-European language converted to prepositions (presumably) in parallel, when such a change contradicts typical word order laws?