Or at least a quick summary! I actually have been asked this several times and I'll just give a few pointers to why I'm not to fond of these guys. (BEFORE I START I DON'T HATE THEM, AND I DON'T HATE YOU FOR USING THEM, THIS IS JUST MY OPINION AND I RESPECT YOURS TO PLEASE RESPECT MINE!)
Well to start off they are overused *puts on hipster glasses* it seems every time I battle some one they have at least one evolution of Eevee. They don't really have the best stats in battle and can't really hold off there own for very long. I can see why Jolteon would be used with its STAB bonus and speed ... but that's just about it! I mean maybe I'm missing something? Is it because the fans love the designs an that there relatively K-9 looking?
Speaking of K-9 looking it seems that a lot of fans flock towards K-9 Pokemon as well! Including
Lucario, Ninetails/ Vulpix, Arcanine/Growlithe, Houndoom, Mightyena, Absol, NOW the new Furfrou theirs probably a ton of other ones but I can't think of at the top of my head. Again I can see why you would choose Lucario and Arcanine (I have one myself!) Because there stats are AWESOME. Still there are so many other Pokemon with just as good stats if not better ... yet here we have HUNDREDS being used. I also wish for them to boost Ninetails up a little, Without its special attack its just another pretty face it seems like! They have to much potential for it, why not give Ninetails a mega? You gave Houndoom one!
ANY WHO Again I'm not saying I hate any of these Pokemon in fact I love a majority of them! I just don't see why so many people flock to the K-9 looking species. There are 649 (NOT INCLUDING X AN Y POKEMON) In the whole series. Branch out a bit people and give some of the other non K-9 Pokemon some love!