According to the Rif, because we Pasken by it over the Yerushalmi.
Even without that, Pashtus is that the Savoraim/Stammaim and Geonim were learning Bavli much more. That’s why our Bavli has a clear Shakla V’tarya and more-or-less standardized Girsaos, which the Yerushalmi clearly doesn’t.