Most great coaches were/are intuitive psychologists. They just _know_ how to read their players, how to convey information, how to motivate and challenge, etc.
The trouble is, _most_ people aren't blessed with that kind of natural ability to read and understand others, and in the past we have had very little to help us understand why some coaches are just so much BETTER than others...even when technical knowledge is held equal.
Why do some coaches keep youth players for years, while others run them off after only one season? Why do some produce tens of collegiate players from their U12 programs, while others produce none? Where do you put limited funds to get the most "bang" for your buck" with kids? Why do kids stay in a sport, and how do you help them get the MOST from the sport while they play it? How do you translate participation in a youth sport in to a lifetime of fitness and health? How do different types of programs (rec vs comp for example) impact long term health results differently?
Research in sports psychology is young, but growing with amazing pace - in part thanks to the incredible obesity trend in the US and elesewhere. It is becoming critical that we understand how to keep kids and adults interested in athletics, and the "coach" is the gatekeeper for most youth sports.
Good solid coaching technique is something that can be studied, understood, and taught to most people. It's not some mysterious magical quality that only certain people have.
The research previously done in tangential fields such as child development, motivation and learning theory, etc., is applicable and is helping sports psych evolve more quickly in recent years.
Given our health issues related to child obesity, and all of the distractions our kids face that pull them away from physical activity...the more we can do to help coaches reach our kids - the better!