Interesting question. For me, it's two things that are related:
1) He got too big. Robinson's game is quickness and flexibility. By bulking up a frame that was already pretty lean and strong, he ended up diminishing what made him special.
2) He never played his ideal position. Shannon made him a DE when he should have been a jack-of-all-trades outside linebacker. Marcus Robinson was a wide receiver in high school and has surprisingly good coverage skills to go along with his pass rush ability. The sad thing is that Golden and D'Onfrio used him precisely in this role throughout camp and were forced to put him back at DE.
Definitely think he will play in the league for a while. His versatility would fit in well on the Patriots.