There are at least two main reasons why probation is the most commonly-used disposition for adjudicated juvenile delinquents. One reason is financial while the other has to do with attitudes towards juvenile offenders and their futures.
On the financial level, probation is a much less expensive way to handle offenders than incarceration is. If we were to incarcerate large numbers of juvenile offenders, we would need more juvenile facilities. We would need to spend money building, staffing, and supplying these facilities. This would cost a great deal of money. Putting an offender on probation is much less expensive.
The other factor has to do with our views of what the juvenile justice system should be trying to achieve. We believe that youthful offenders are, for the most part, able to be rehabilitated. We want to sanction them in ways that will allow them to become productive members of society. We believe that incarceration is not conducive to achieving this goal. We do not want to put large numbers of juveniles in facilities where they might become hardened criminals. We do not want to take them away from family support systems and regular schools. Therefore, we use probation as a sanction against them. We hope that it will be severe enough to move them away from crime but lenient enough so that it will not reduce their prospects for success in the future.