The popular guess is that Johan wants people who are willing to put in work, and he wants people that want the Mosconi cup bad and have that burning desire. Justin made essentially no effort to even attempt to get the points to qualify to make the team, and this is seen as a demonstration of not having either of those things.
I think it was also intended to send a message to everyone for the future, the message being that you better be willing to put in some work and effort and be acting like you want it and that it means something to you and that you aren't going to get a free pass without those things regardless or who you are or how good you are.
Money wise Justin made a poor business decision because he could have gone to a whole lot more tournaments and still come out ahead after he got the minimum $10,000 show up money just for making the team. Justin isn't even the type of player that is going to lose money on average by playing more events, especially all the stateside events, but even if playing some more events did end up putting him a little in the negative that $10,000-20,000 Mosconi payday would more than cover it and then some.