Because every service and every component, and every branch contributes to those operations. COIN is the same way. I think this is counter-intuitive but if we want COIN to be equal to Major Combat Operations then we should NOT have a single proponent because once we do that we allow abdication of responsibility for it to studied and practiced by all the organizations that are not the proponent.
One of the problems I think we have with Foreign Internal Defense (FID) (and we should acknowledge that COIN is a part of FID but that is another discussion) has been that the Nunn-Cohen act established FID as a SOF Core mission in the law. Because of that there is a perception that FID is a SOF exclusive mission when in fact all services have always participated in FID in a variety of ways. But because FID is perceived as a SOF core mission few outside of SOF have given it much thought. And the same will be true (and has been true) of COIN. No one really gave it much thought until post 2003 (except those of us who have been practicing it and studying it for many years before) Now COIN has become the new popular "shiny toy" to chase after and there are many that want to jump on the band wagon and as we have seen recreate the wheel. Let's be frank, there is very little in the COIN realm that we are doing or trying that hasn't been done or tried before. It is a question of understanding the tools and organizations, and tactics, techniques, and procedures available combined with the proper assessment of conditions and then the correct application of those tools (and an understanding that we may not get it right the first time or achieve the effects we thought we wanted to achieve and we have to have the strength and confidence to get over such "failure" and adapt to complex and changing situations). We have people who think they are discovering something new when in fact as JFK said in 1961 it is "ancient in its origins"... but requiring ..."whole new kind of strategy".
We have to find the right balance of capabilities to conduct the full spectrum of operations. That is the hard task. If you want COIN to be on the same level with MCO (or perhaps not even on the same level, but on the right level with MCO) then it is going to require OSD, JCS, and Service direction and putting the right emphasis on it in our education and training systems and in our doctrine and force structure development and personnel assignment systems. No one proponent can achieve that across the services. Too often we equate COIN and FID with simply train, advise, and assist and we want to establish a school for training advisers and we think that that will answer the mail on COIN, FID, etc. But as we all know COIN is so much more than just training and advising indigenous forces. But by giving proponency to one service or agency we will never get buy in from all the services and agencies that need to conduct it. The feeling will be I am glad that service or agency had the rose pinned on them, now I don't have to worry about it. But COIN (and FID) is too important to leave it to a single proponent.
And then I would take issue with the people we are fighting the COIN fight in Iraq and Afghanistan and that we have to win. We should not be fighting two active insurgencies as if we are the main effort. We have to be the supporting effort or the country in which we are conducting those supporting operations will never achieve success. We have to get out of the mindset of achieving US success. Our interests will be not achieved unless our friend, partner, or ally is successful in defeating its threats or eradicating the conditions that cause those threats to emerge. The Afghans and Iraqis have to win (it remains our responsibility to help them win but it is no longer our fight to win -- we did that in 2001 in Afghanistan and in 2003 in Iraq).
It would be better stated that we are supporting COIN operations of friends, partner, and allies by conducting Combat FID in 2 countries of one theater (Iraq and Afghanistan) and we are conducting Direct FID in multiple other countries/regions (e.g., Colombia, Philippines, Trans-Sahel, Horn of Africa, etc) and we are conducting Indirect FID (security assistance, IMET, training or MTTs, small unit exchanges or partner to partner/military to military engagement all to help build capacity of friends partners and allies) ideally to help prevent an insurgency, or terrorism, or internal instability, or trans-regional threats, or ungoverned spaces. We have the doctrine, we have the tools and organizations, we have the people and even the expertise. What I think is really lacking is not a proponent but a real strategy that will determine the end(s) or at least the objectives we are trying to achieve globally, regionally, and even on a country by country basis (a Theater Security Cooperation Plan??) then application of our resources in ways to achieve those ends. A strategy is the key to bringing all this together in my humble opinion.