A BOI Tower/TRACON Controller reported Supervisors routinely ignore Controller's request to de-combine radar sectors during busy traffic periods; even though there is sufficient staffing to open the position.

Narrative

I am writing this to report what many controllers at BOI believe is a very serious safety issue that is being ignored by management at this facility and at higher levels. When I got to work yesterday; there was only 1 controller working both Boise Radar sectors. Traffic levels were high and complex. The controller on position told me that he NEEDED a radar split due to traffic levels. He also told me that he had asked the Supervisor on duty to send him some help and that a sector split was needed for traffic volume. The Supervisor on duty was not willing to split the radar sectors because they were more concerned with accomplishing training in the Tower. There were multiple trainees being trained in the Tower at the time of this issue. The Supervisor could easily have stopped training one of the trainees and sent someone to help with radar traffic.Obviously; this is serious safety issue. Management at this facility continually chooses to keep training positions open so that they can meet the 3 hour training mandate instead of focusing on safety of aircraft. This is causing huge amounts of stress and fatigue on the controllers at BOI. We all understand that training must be accomplished. But you have situations where one controller is working the entire Boise airspace alone; asking for help; and the Supervisor is unwilling send help. This practice must come to an end. When controllers ask for help; especially in the radar room where they are typically working alone; management must prioritize this over training. We've seen over and over again where a Controller in Charge will stop training due to traffic volume; and end up having the Manager or Supervisor override that decision. This is unsafe for everyone involved.