Why would an inmate who violates probation get sent back to prison?

The actual determination of being sent back to jail or not lis with the judge who presided over the original case. This is where the word of the probation officer has little weight. They are not the final decision maker. They may very well said, "I'll recommend to the judge to please defer sentencing in lieu of enrollment to a rehab center." The judge can take the recommendation or not. In this case, maybe there was something bothering the judge and did not feel comfortable with the recommendation.