Why? Past production doesn't correlate to new production in a new system. While he found some playing time under Mike Malone, Casspi didn't really flourish until all hell broke loose and Ty Corbin and George Karl threw out the defense first mindset for score first anarchy.
Omri tries hard, yes. He also tends to have massive Francisco Garcia level brainfarts and this season has proven incapable of shooting lay-ups, three-pointers, and playing defense. That isn't to say he's a bad player but Dave Joerger has said time and time again that you have to play defense to see time in his rotation, which, let's be honest, features better defenders that can play the same position as Casspi. Unless you want to make the odd argument that Omri is somehow a better defender than Matt Barnes, that's already twenty minutes of potential playing time for Omri gone.
But saying Omri is better than Matt Barnes? Matt Barnes has been one of the better defenders in the league for a decade. He's a veteran of Joerger's system, rebounds well for his position at this point in his career, and, for all that bluster about Omri being the best shooter on the team, is actually willing to shoot from the three point line (in all fairness to Omri, he's not a great spot-up shooter) as opposed to Omri's sudden reluctance to not shoot from the perimeter. Matt Barnes is a good basketball player and he's definitely earned his time on the court.
How about at the 4 spot? Well, Joerger's old school in that he generally tends to play two bigs at a time, though this season he's also stuck Barnes in there at times, which he can do because Barnes is capable of holding his own at the four. Again, if you want to make the argument that Omri is a better defender than Matt Barnes, go ahead.
If you want to say Omri is a better player than Tolliver, you probably have an argument but Tolliver isn't exactly seeing all that much playing time to begin with and when he does play, he generally plays within himself and avoids doing anything too stupid. Omri, thus far in the season, has not.
Does Omri deserve playing time? Yes. But he has to earn it.