From a design point of view, I think the ears are stylized to make it easier to put rings on them. Umbreon is a generation two pokemon, so pokemon designs had to deal with the limitations of the game boy/game boy color, which only had so many colors (black, white, and two additional colors) and only so much room in a sprite you could use for a pokemon (they are rather small). While not impossible to make the ears like other eeveelutions in sprite form, it'd would be rather difficult and would likely make the ears rather visually cluttered.
Of course this is all a big assumption on my part and is likely not the only reason if my assumption is correct.
Edit: Fixed a typo