1. Lack of trust from coaching staff
It became apparent relatively early in the season that head coach Frank Vogel and his coaching staff did not quite trust Watanabe. Perhaps that is because he needs to be hidden defensively on some nights, although Watanabe needs only look at Bol Bol as an example of a player who adapted to playing with the Suns this season.
Bol barely played to begin the campaign, but at this point he gets some minutes and he takes them well by being a weird matchup for opponents. Defensively he is still dreadful - and unfortunately for Watanabe he does not possess a body like Bol's either - but he did enough to gain the trust of coach Vogel, meaning Watanabe had the chance to do the same thing.
He did average around 18 minutes per game through November and December - but as soon as Beal got back from injury properly - Watanabe's minutes began to vanish. There is no shame in losing your spot to Beal, or Allen for that matter given the season he has had, but these circumstances all happening for Watanabe at the same time were unfortunate.
About as good as it ever got for the player was shooting 43.8 percent through the first four games of the season, when it looked like he hit the ground running. The Suns started out 2-2, coach Vogel tinkered with some rotations to try and fix this, and Watanabe' confidence seemed to vanish at a rapid rate. The hope now is that he can get back on track with the struggling Grizzlies.