1. The works were still in their original Greek (rather than translations into Latin done long ago, or the translations from Greek to Arabic (and then to Latin) that came in through moorish Spain).
2. Religious attitudes allowed the works to be studied close to their original form. Early Christians were basically of the opinion that if a book disagreed with the bible it was heresy and should be destroyed, and if it agreed with the bible it was unnecessary and could be destroyed. That was less the case in the eastern side of the church and of course a big exaggeration too: there were old Latin translations of eg Plato however they had been quite bastardised to conform with Christian theology (I understand Aristotle was a little less bastardised as early Christian theology was itself quite Aristotelian). By the renaissance less bastardisation was required (if translating to Latin rather than the vernacular).
Without being transferred so late to the west, the works may never have made it to modern times in such a complete form.