Quick Charge is supported by the Qualcomm Chipset, so potentially going to be implemented and standardised between different manufacturers, allowing for compatible cables and chargers.
That is not the case with Dash Charge which is proprietary and unique to OnePlus devices and accessories.
Dash Charge takes the work, the power management away from the phone and puts it into the charger and cable, hence that difference.
That means the phone doesn't heat up additionally while still being used while charging, and doesn't need to throttle performance to deal with the extra heat as a result.
OnePlus can use other accessories, but it just won't use its own Dash Charge, nor does it support Quick Charge at all.
Standardised accessories, vs an arguably superior version.