Various reasons. A lot comes down to the individual facility requirements and there are special needs at each. LOAs to memorize, and unique challenges like with PCTs control of airspace around DC where they run extremely compressed finals to fit all the traffic.
ZHU is also notoriously hard for some (weather is a big factor) and then you do ofc have some not so great historic trends of intentionally trying to wash trainees so OT can be guarded for the CPCs (N90 was infamously known for this but seems to have quelled in latest years)
Shortly said the job is not cookie cutter even after the academy and you need to certify on all positions to be a CPC at your chosen facility. There's levels to each btw up to 12 being the busiest. When you wash there's a chance to retrain and go to another facility as well and there are NATCA (controller union) approved processes to transfer, but yeah many just change jobs after that.
ZHU is also notoriously hard for some (weather is a big factor) and then you do ofc have some not so great historic trends of intentionally trying to wash trainees so OT can be guarded for the CPCs (N90 was infamously known for this but seems to have quelled in latest years)
Shortly said the job is not cookie cutter even after the academy and you need to certify on all positions to be a CPC at your chosen facility. There's levels to each btw up to 12 being the busiest. When you wash there's a chance to retrain and go to another facility as well and there are NATCA (controller union) approved processes to transfer, but yeah many just change jobs after that.