The USMC decided that accurate fire in lesser volumes was more important to the type of fighting occured these days.
The M249/Mk46/C9 etc are a disposable machine gun, they have a very limited lifecycle, and the bodies wearout.
We at KAC make the Stoner LMG (SLMG), which is not really a LMG more of an assault MG - its been adopted by some people - Danish Naval SF being an open source user group.
Our IAR candidate as well as LWRCI and others did not make the cut, Colt, FN, and Hk being downselected for the eventual adoption of one model.
I for one don't agree with the concept of the IAR (but I do with the AMG concept of the SLMG), as I think that while the IAR works well for Urban work in an COIN environment where low volumes of fire are necessitated due to non combatants being in the area, it is not a good platform for a less restricted area of operations and small units where a high volume of fire can be necessary for survival..