I think there are multiple items at play here. I think he is seeing a lot of competition from his teammates for catches. His stats for 2015 and 2016 are below:
2015 59 catches for 916 yds and 7 TD
2016 56 catches for 840 yds and 7 TD
You also have to remember these 2016 stats were bolstered by a 13 catch 238 yd performance against Wake this year.
I think he sees the writing on the wall that he may be passed up. I don't think he feels staying has a ton of potential for him to be better than this year.
This draft is a pretty weak draft at the receiver position. Similar to the QB position, and hence all the questionable guys coming out, this draft is pretty thin at the receiver postion.
I think he could benefit from another year to get his routes sharper and progress, but who knows maybe he falls into the right position and can succeed. The one thing that he does do well, in my opinion, is block downfield. I think he could be a better route runner, but if that has not gotten better at this point maybe it never will. I don't think it hurts FSU all that much, although you always hate to lose depth.