Not sure what you mean by this, only thing that really changed from MK9 to MKX is the gameplay/mechanics, and that's has nothing to do with how successful the game is! MKX's mainstream success came from its excellent value for casuals, and maybe art direction and gore, which are pretty much inline with what they did with MK9!
No, MKX art direction has nothing to do with MK9. MK9 went the SF4 road on the retro-trip who bears his heritage. So they went full speed on this road: all the old characters, ninjas suits, female dressed as strippers, cheap aesthetics. The hero is Bruce Lee/Liu Kang with Sonya and Johnny Cage/Van Dame joining. There are many things I could detail like the look of the character select screen, the fonts, the life bars... But the goal was to appeal to people that played MK in the nineties.
In MKX they went for a more realistic tone, inspired by dark comics and more recent references. Female characters finally are dressed with clothes defining them as full fledged characters (Kitana and Mileena redesigns are great and helps you identify how they are different despite being sisters). It's also a game with different body types, different monsters/races that are new to the MK universe. They also redesigned everything from the character select screen to the lifebars, the menus, to give them a more mature and classy look.
Why? Because they understood quickly they could not sell the same thing twice. They needed a fresh take on MK so they did it. There's a new team of younger characters that references new tropes, new attitudes and is very diverse: a white woman, a black woman, two asians guys. They aged some of their most famous characters like Liu Kang and Kung Lao and that was a good decision because these references are OLD. If you were born in 2000 you're sixteen today. And the most famous martial art movies that came out since you are born is probably Crouching Tiger, Hero and... Kill Bill ? Movies like IP Man, The Raid or Ong Bak aren't that famous.
You probably never saw a Bruce Lee movie if nobody is a fan in the family. Jackie Chan is the old asian guy in Rush Hour. Van Damme is the guy in the World of Warcraft ad. What's the point to keep main characters inspired by them if you want to sell a game to teenagers and young adults? You have to create a new aesthetic, new younger characters with backgrounds and stories that young people of today can relate to. You can keep old chars for the old fans but that's not the selling point. Remember that Johnny, Cassie or Jax were among the last chars to be presented.
That's what Tekken 3 did, putting chars like Baek and Kazuya to the trash to replace them with Hwoarang and Jin. They changed the aesthetics too, did a packed game and it sold 8 millions. MKX did the same thing last year and is probably at 6 millions copies sold today.
Aesthetics, design is the first thing you notice and it starts with the art on the box. Now compare these and you'll understand that those two games, despite being the same franchise, aren't aimed to the same exact public.
Edit : Something that NRS had and that Capcom hasn't is also a duo of characters (Scorpion and Sub) that are both the icons of the series but not the heroes of the series. They'll always be here and they are you attach point. But neither of them is the hero so their aesthetics or story or gameplay will not hold back the series like Ryu is doing in SF.
It means that NRS can create a new MK with new heroes anytime, Scorpion and Sub will always be here. Capcom is stuck with a karate guy in a pyjama living like a hobo with anger problems. They should put Ryu and Chun/Ken as their icons too and create new characters for Street Fighter to replace them as heroes. This way you can attract a new public while still pleasing old fans.
Also NRS was smart with their character unveils: they showed scorpion and sub first, then Cassie, then three new monster characters (Dvora, Kotal, Ferra Tor) so people knew that: 1. Scop/Sub are here 2. We'll see fresh takes on favorites chars 3. Many new chars to discover, following the game is gonna be interesting. Meanwhile at Capcom : Ryu, Chun, a not so fresh take on Nash (except for gameplay), Bison, Cammy... Too few surprises.