An answer from my sister (an f(x)/EXO stan) on Tumblr on this manner:

It is incredibly unfair in terms of how many years they have in comparison, but you do have to acknowledge the explosion of EXO’s popularity makes the idea of a concert seem a lot more profitable to SM, and at the end of the day, It’s not about the fans or the entertainment, but the company’s earned revenue.  There is not a lack of entertainment, there’s an excess in terms of the Kpop world, and with all that competition, even if it’s unfair to f(x), SM would want to remain several steps ahead of the rest in order to prolong their legacies in the event of a dip in the overall popularity of kpop.

And it’s not like a concert is an easy thing, it’s a good deal of work for the idols too.  EXO deserves all the love and favouritism they get.

I kind of agree on that. It is extremely unfair for F(x) not being able to have a concert after so long. But then again, to every single company, it's all about the revenue. And considering SM's over-hyping of EXO, they feel as if due to that, they believe that they can profit the most from them rather than F(x). Aside from the fairness issue, I do have to side with SM on the profiting side of the debate. SM's marketing techniques are seriously off of the charts. They KNOW how to milk money and attention. They resort to even establishing scandals for their own artists. They set up that Yoona/Psy scandal to bring back attention towards SNSD before their IGAB comeback. The fact that the scandal seemed so fake and unbelieveable made people believe, "Why now? Is SNSD having a comeback soon or something?" And it freaking worked. IGAB soon came out and it blew up.

After EXO's MAMA debut, SM kept having the members travel across airport to airport, giving off an elitist image, keeping the fans craving for them nonstop.  And don't forget, EXO had just sold over 1,000,000 albums. They know how to market. And that's all that matters to them. The profiting aspect of the industry. They don't care about the fans at that point. And the worst part about that is that the fans will ALWAYS buy into it.

Actually, EVERYTHING plays into EXO getting a concert early on. The insane fans bring a horrible rep to the fans and the idol group so the sane fans will try to negate that by hosting donations, buying their merchandise, and etc. to show their idols that there are actually people supporting/loving them. EVERYTHING plays in. From the haters to the company and to the fanclub. Opponents/haters in my opinion are frankly fueling the fire since the EXO stans will often retaliate and then campaigns will pop up all over the media to help combat that negativity in trying to show as much support as they can for EXO (i.e. the 5 M view goal for EXO's new MV).

That specific ad to be exact:

So in short, although it is extremely unfair, it's SM. Remember though, no company is perfect. SM is not perfect, YG is not perfect, JYP is not perfect, TS, Woolim, and etc. They ALL have their flaws that make them more disliked and liked in certain categories. F(x) stans, if you want to get F(x) a concert soon, then continue supporting the group. SM will have to give one to them by the end of their career otherwise, all hell will break loose for them. It'll be tough, but I know that it will happen so don't lose hope.

Haters/Opponents of EXO's popularity, if you don't want EXO to have so many things at once, STOP PICKING ON THEM AND THEIR FANCLUB AND GO SUPPORT YOUR OWN BIAS GROUPS. Cause seriously, you're just feeding into the industry (look at picture above) even more given how new the group and fans are in the industry. You're just making your cause suffer by making SM bias towards EXO even more. Not only will it affect you but pretty much ALL of the Kpop fans as SM will primarily focus on EXO, "overshadowing" a huge chunk of Kpop groups in the process. You fans say you're fighting against that but in reality, you're just making it worse from what I'm seeing.

Rant over.