I would place the blame as follows...
LeBron - because he doesn't play off the ball enough to suck defenders away from Love. When he is off-ball he's standing 2 feet behind the 3pt line completely out of the play.
Lue - because his offense is basically drive and kick and iso. And he hasn't put Love in the right positions to be successful, such as elbow touches or dribble hand-offs where he can either pop, roll, or keep the dribble and drive. Instead he puts Love in the low post once or twice a game where he's simply not as effective anymore because he's lost a ton of weight, the game has changed, and Tristan is there clogging the paint.
Kyrie - needs no explaining.
Love - Contrary to the popular opinion I think he should have never accepted his role and allowed himself to become so marginalized. If anyone should've diminished their game to accommodate the other star players it should've been Kyrie. Compared to Kyrie, Love was arguably the more accomplished player prior to 2015 and had more success carrying his team in a deeper western conference.
Again I would blame Bron for Kyrie getting more touches than Love because Bron loves to have that other guy he can dump the ball of to for a whole possession and exit the play so he can take a break. He did it at least 5 or 6 times a game with Wade and you saw it all the time with Mo Williams back in the day.
The Front office - for not putting the right players around him. We need more good cutters and finishers...players who can play without the ball. Crowder should help in that area tremendously, hopefully Jeff Green too. And it cannot be stated enough how much it hurts not having a pass first point guard. Delly did wonders for this team for that reason imagine if we had someone who was a slight upgrade to him. Can we reincarnate Jason Kidd?