I am a coach and VP of operations for the girls side, and DH is a coach, plus the director of coaches and player safety for the boys side, for one of our local sports organizations. We live and breathe this subject for much of the year.
In my opinion, being a coachable kid means coming to practice with a willingness to give 100% and a true belief that being at practice on any given day is the very best place to be. The child has to believe that they are part of a team - a family who looks out for each other and cares about each other. I never look for the best - I look for the ones who have the best attitude and give the most effort.
This starts with the coaches. The coaches have a responsibility to the children to sell the idea of being a team and giving 100% every play, every day.
Little story - my DH was asked to coach a 8th grade silver team last year. He was already helping coach our little DS's team, and DS15 had already moved onto high school, so we didn't have a child at that level. But the league needed a coach so DH said yes. He assembled a coaching staff and went to the first week of practice. The year before, the silver team lost every game and scored a total of 2 touchdowns the whole season. The current 8th grade coach decided to run a "combine" to see who he wanted on his Gold team, and at the end of the week, called out the names of the kids who made his team. The rest of the kids sat in the grass and listened to the Gold Coach tell them: "Now, don't worry if you didn't make the gold team - these guys are just better and bigger. It doesn't mean you are bad players, it just means that you aren't as good as those other boys" (or something to that effect). These poor kids walked over to DH with zero confidence and heads hanging low.
DH saw this and decided that he needed to do something FAST or these kids would believe they were no good. He called all the kids over and said: "Everyone look at me and listen very carefully. WE are a TEAM. We are a FAMILY. We are second to NOBODY. I can't promise much but this I can tell you, right here and now: If you stay, you will be a champion. I will do my best every day to coach you and get you ready to play high school ball and if you stick with me and believe, you will walk off the field at the end of the season a champion."
These boys believed. They trusted their coaches. They bought in. They knew their coaches had their backs. Kid after kid after kid on the Gold team asked their parents if they could move to the Silver. Parents of Gold players wanted their kid on Silver. The Gold coaches were bitter and angry LOL and couldn't figure out why their "elite" athletes wanted to play down. They just didn't get it. While they were yelling and screaming at their boys, my DH was COACHING his - he was showing his boys what to do, explaining why, using high school players to demo plays, stances, etc. He got to them on their level and always treated every player with respect and encouragement to do their best. He didn't make them run laps or use punishments for no reason - if they played a position where running was necessary, they ran. If they didn't, they did something else. Every thing he did was based on what would improve the player.
These kids went out and played their heart out in every game for the coaches that believed in them from the beginning. They worked their hardest to prove those Gold coaches wrong and to show them what they could do. They won almost every game by a decisive final score, ending the regular season 8-1, while the "gold" team went 1-8. Our boys went onto the playoffs and won the first round. They led the whole game but lost the 2nd round game (that would've sent them to the championship) literally in the last 2 minutes on a fumble that the other team recovered and ran back for a touchdown for a final score of 18-16. The heartbreak and emotion of those boys in those moments after the game who believed in themselves so much to get that far moved the whole stadium to tears. Even the coaches on the other team came over to talk to the boys with tears in their eyes and said they have never played a team like ours and could see the brotherhood and heart they exibited both on the field and off.
DH cried through his final speech and told the boys that the gift they gave HIM was more than he could have ever dreamed of, and that even though they didn't get the trophy, they were ALL walking off the field as champions because sometimes as hard as you work for the trophy, the trophy is not the end goal - it is the character, confidence, and relationships you build that are the true reward.
DH has written several letters of reccomendation for these boys who are being recruited by Private high schools for the fall

He has gotten letter after letter from various parents saying unbelievably nice things, and his starting quarterback, who wasn't "good" enough to make the gold team, will be the starting Freshman QB at their HS in the fall. The Gold QB will be his backup.
So, long story short - if the adults involved can get a kid to buy into the belief that what they are doing is worth it - they can be a "coachable" kid. They have to trust you and know you are giving them your everything and they will follow.