There has been no need to port essentialsX. This is mainly because there is a plugin with almost the exact same commands and sintax called Nucleus.
I know some plugins offer reading/converting of bukkit plugin files to sponge, not sure if nucleus has essentialsX support.
But as mentioned before, because there is already a well know plugin (every sponge server i know uses it) that has all the features, its not worth the time and energy to port another other. It would be simpiler to add the reading of bukkit essentialsx files to Nucleus (if it doesnt already have that support)