Because it's quite a bit cheaper than an 8000, even if the 8000 is configured exactly the same as a 6000 might be (i.e. two of the bands deleted, making it a single-band radio). When you order an 8000, you are still paying for the RF hardware to support all bands, which is always there even if not enabled in the FLASH.
NYS contract pricing (base model, no features added)
APX 8000 All Band Model 3.5 Portable $4,719
APX 8000 Single Band Model 3.5 Portable $3,119
APX 6000 700/800 Model 3.5 Portable $2,591
That $528 difference between a single band 8000 and single band 6000 might not seem like much, but it's $52,000 for every 100 radios purchased, or half a million dollars for every 1,000 radios purchased. If your agency saves $52,000 by purchasing 100 of the 6000, that's 20 extra radios worth of savings over the 8000. It adds up quickly.