Defense is not just speed or vert or length. Not everyone has the defensive instincts to play at this level. I don't know what else to call it, but Johnson just doesn't have it. 9 fouls in 27 minutes in two blowout wins against Sacred Heart and UNH. These are stat-padder games where Johnson should go in with the mindset of getting 10 blocks, and he fouls out of one and almost fouls out of another.
Okafor was on the far extreme, but his defensive instincts and sense of space, timing and how to defend were amazing. There are better centers in the modern era, but there are probably not 5 college centers in the last 30 years with better defensive instincts than Okafor. There may not be 2 or 3. Johnson doesn't seem to have any defensive instincts at all.