Back in August 2019 Ed Boon announced that Mortal Kombat 2011 had sold close to 6 million copies. What a big turn around for a previously declining series. Then  he also announced that Mortal Kombat X sold 11 million copies. Wow, that is nearly doubling the previous game's sales. That is a crazy increase for a sequel especially for the fighting genre. Currently Mortal Kombat 11 is at 8 million and will no doubt be at least do 10 million. 

Fighting games have made a comeback in the online age of gaming but are still relatively niche in regards to other genres. Netherrealm looks like they have a winning formula though. What do you guys think Mortal Kombat is doing right and what can other fighting games learn from it?

Well as the 90s and early 2000s taught us. Don't try to be a clone of MK or copy the fatalities. You will fail. I think MK is just more fun casually because of the brutality of it. It can be played by more hardcore but since I grew up since the series was new, it was always just having fun with the gore. Other tried to copy it but MK was still a very competent fighting game series and its gore is creative. Characters know how to mix the right amount of seriousness with the over the top goof shit it is.

I think it comes down to how well known the franchise is along with how casual friendly it is. Smash Bros. is the best selling fighting game franchise ever because it is casual friendly and the characters are recognizable. It's the same with Dragon Ball games. Those sell well because everyone knows Dragon Ball.

Mortal kombat games are full games in release : Single player and multiplayer modes are robust and have a good quantity of characters.
You can play with friends just for fun or in a competitive way with harcore gamers.

Street Figther 5 Was a reallly barebones game, and everyone knows Capcom are going to release a lot of versions or revision of the same game. Not worth buying that in release.

Guilty Gear is too much focused in the harcore guilty gear fan. Casuals have no reason to learn the game.

Tekken and Injustice are another popular and very complete fighting games .

MK11 sells itself for $15 and then charges massively for microtransactions/DLC. It's a series that is about 30 years old, so lots of lapsed fans can easily come back to it.

Dragonball FighterZ has sold over 6 million copies on the same $15 sale premise, except without the microtransactions. DBZ doesn't have as wide appeal as MK though.

Curious why you are bringing up MK11 as $15? That is a very very skewed view.

1) MK11 selling over 8 million accounts for more than America and the $15 price tag is only in America

2) MK11 has had multiple "new" releases all of which were and are well over $15,

Standard Edition

Premium Edition

Special Edition

Aftermath Kollection

Ultimate

3) MK11 at $15 is only the base standard edition and when it's on sale. 

4) MK11 at 8 million was four months ago, before Christmas

It's as if you are trying to bring up the $15 as a means of saying the game even though it has sold over 8 million really didn't make that much money.

Let's be fair and honest, Mortal Kombat has made a massive combat last generation and now is the most successful fighting game franchise, passing up street fighter as the worlds most successful fighting game with over 45 million in sales.

MK was overrated but interesting in the 90s. It was amazing because it pushed the envelope of blood and gore in the industry. However the mechanics were very bad. The SNES version had so much delay with the attacks and the hit boxes were so inaccurate its laughable.
Many games came out after that polished and improved upon them. In my opinion, KI on the SNES (tho wasnt perfecf) pushed the fighting genre forward due to its speed and combos that its now a standard among most fighting games. Go play the originals of Street Fighter 2 and MK and see how slow they are to play where as KI was all those games on crack and still offered Blood and finishers aswell.

I think single player content has a lot to do with it. Mortal Kombat games, along with the Injustice titles, tend to have fairly lengthy story modes, lasting over twice as long as what you'll find in games like Street Fighter. There's a sizable minority of people who spend most of their time with a fighter playing single player, and Netherrealm caters to them.