Checking may not be all, but it has a lot to do with it. Some kids who will grow and be over six foot don't reach full height until after 18 years old (my father for one -- 5' 9" as a senior in high school and 6'1" at 20 years old).
Basically the way the system is set up now with larger and early birthday kids more likely to make higher level teams, you're hoping that the larger/oldr kids at 13 and 14 yo are going to be your best players at 18 years old. That, however, is not always the case and a lot of potential talent is lost when holding off checking a few years will give more kids time to grow, time to develop skills and stick with the game. Bantam, statistically, is where injuries -- head injuries especially -- begin to jump for kids. One concussion predisposes kids to more concussions and each one potentially more difficult to recover from, so that by 15 and 16, they've had too many injuries to continue on -- often in any contact sport and for good.
Taking checking out of 14U at least would give kids two more years to get through or begin puberty, as well as time to continue to develop skills. Research shows that learning to check earlier (12U) had little to no protective benefit at 14U and only increased the number of head injuries at 12U.
Hockey is a late peaking sport, and yet by 16 years old, kids are dropping out not only because they don't think they'll make it to the NHL, but because they've already suffered too many injuries and need to weigh brain health and body health with academics, which is really their primary job as a child.