Jamison suggested that despite both of their immense talents, the team (players and coaches) were simply unable to get them on the same page.
Per ESPN Los Angeles:
What Jamison never figured out though, was why the relationship between Bryant and Howard never developed on the court.
“It was difficult,” he said. “I really believed before we got started that those two could co-exist. But it just didn’t work out that way. Both guys are unbelievable basketball players, the best we have in the game right now. But for whatever reason we just couldn’t get that relationship as far as them communicating and them trusting each other the way you needed them to trust each other in order for us to win a championship. It just didn’t happen that way.”
Bryant and Howard had issues with teammates long before they ever became teammates, so, their inability to work together shouldn’t be much of a shock.