For reasons I've mentioned above. The 'special-type'-group came to stop existing in GenIV, yet, Gamefreak chose the Ice and Grass-types for their next Eeveelutions, when they could've gone with any other type. Leaving only one type from that former group, the Dragon-type. Now they issue a single Eeveelution. Remember, they already had the chance to cover whatever type they wanted, but chose from that specific group. How stupid would it be if they didn't choose from that group this time as well? I mean if they wanted a Fighting-type Eeveelution, then they could've done that back in GenIV
already.. The seven previous Eeveelutions in a sense vouch for the Dragon-type to be included.