Koushiro was in the school's soccer
club with Taichi and Sora (this might extend to with Hikari and Daisuke, too). Clubs have regular tryouts during the school season to determine the members that will be playing for the school's soccer
team. So technically you can be a club member but not be a team member.
This bit about him was mentioned in his extended character sheet profile by Digimon writer/translater Meg-chan on her Digimon Sekai website.
In 02, however, it's mentioned that Koushiro was in the computer club. It's possible he was in two clubs, and it's also possible that the writing staff's decision in having him be part of the soccer club has been retconned (kinda unlikely since he and Taichi showed a pretty strong friendship during the entire first season). Either way, the likeliest solution is simply that after his experiences in the Digital World Koushiro decided to pursue his own interests over the interests of his friends, and left the soccer club for the computer club. I say this because while Daisuke remembers Taichi from the soccer club, he knows next to nothing about Koushiro or Sora, who were both in the soccer club along with Taichi. So you can assume that Sora left for the tennis club and Koushiro left for the computer club before Daisuke decided to join the soccer club.
An interesting tidbit is that even Daisuke wasn't a regular member of the soccer team. This means he doesn't play in every match the school has and that his game against Ken was probably one of the only ones he played on the field. He only becomes a regular in the 6th grade (he tries to brag about it on one of the Drama-CD's, but becomes saddened when he reminds himself that Taichi became a regular team member by the 5th grade).
So yeah, sporting clubs have tough competition. So just because there's a chance Koushiro wasn't a regular member, this doesn't also mean he was a bad player (Daisuke being the perfect example of this).
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