I read a theory on the UG when I was very new to the forum around 08 and it has always stuck with me, as I believe it has come true. Grappling defence is easier to learn than grappling offence and conversely that striking defence is harder to learn than striking offence. This gives people of lesser grappling skill, the ability to defend themselves grappling against higher level guys, but in striking the better striker will have better defence(usually), leading him to be hit less and hit the other guy more. Once you have a situation like this where it easier to damage someone in one aspect than the other, guys have more success with it, in training and the fight, giving a confirmation bias towards that aspect - thereby they devote more time to training it and gameplan around it - which in turn only compounds the issue.
So now you have guys who are having more success striking than grappling due to the inherent learning curves in the respective aspects, devoting more of their time to training for one aspect other the other and then it just goes round and round in a circle, until you end up with almost Ultimate Kickboxing. However, grappling is still an important part of the game and will never go away, since some guys will always buck the trend and you at least need that defence.