Is it possible that Melinda could be behind this? Sunflower and Surf's Up are considered to be some of the best group albums, with everyone getting a chance to shine. This goes against the "Brian is the sole genius of the Beach Boys" narrative, so I could see Melinda having a problem with a box set covering this period. From Mike's book, we know she interfered with the C50 tour by changing a setlist and ordering five autotune devices. If she can do that, it seems likely she can get a box set delayed.
Maybe she doesn't like the tracklist. There may not be room for both 'Til I Die (track and backing vocals) and Student Demonstration Time (alternate vocal take) and I could see her lobbying for the former.
I'm just speculating and there's no evidence for this, but it seems plausible to me.