Why not?!
Lack of talent.
How many players do we have that are top five players at their position? None.
How many that are top ten players at their position? None.
How many players that are above average starters? Lee, maybe.
The team is lacking in high end talent. That is why they are not a contender.
How many players do we have that are average starters?
This is where I'd put Lee. You could debate Amir Johnson if he was playing PF still (but not at center position; too undersized to make up for low skill-level). Depends how highly you rate Bradley (SG position is weakest in the league). And whether you think Marcus Smart can make a leap next year (I am looking more to year 3 myself).
What is the rest of the team? Below average starters to high quality bench players.