That's because the approaches and the connections to the road network are the state's responsibility, not USACE which only owns the canal and the bridges themselves. MassDOT released its own report on this, which I haven't read. https://www.mass.gov/lists/cape-cod-canal-study-documents
Skimming the executive summary, looks they're contemplating major redesigns of pretty much the whole set of roadways/intersections/interchanges surrounding the canal, potentially including completely nuking the Bourne Rotary for a real interchange, among other things.
Interesting stuff in there, and worth a read.