Because a Box Mod device is basically a glorified battery. Even the fancy ones with OLED screens are simply there to control how much energy is sent to the heater.
The problem is there are simply no "brains" inside a box mod to regulate temperature. A dry herb atomizer would basically just keep cooking until the temperature exceeds the combustion temp and the herb combusts. There is nothing telling the heater when to shut off or stop heating up except the user pressing the power button to send juice to the heater.
"But wait! Aren't there temperature controlled Ecigs on the market using a box mod??" Why yes little timmy, you are correct! Box mods do have one fancy trick up their sleeve - they can measure the resistance of the heater. Hotter wires have less resistance than cold ones (or vice versa, I forget). Basically a temperature controlled E-Cig is not actually using a "temperature probe" to monitor temperature, instead it uses a mathematical formula to calculate the temperature based on the voltage supplied to the heater, and the current resistance of the heater.
The problem is these temperature regulators don't work properly with dry herb atomizers (the math gets all fucked up due to the different heater), so you can't use that method for controlling temps (unless you hack the firmware of the boxmod and update the calculations with the proper info about your heater. But even then doubtful.
So basically it comes down to temperatures and regulating them so you don't accidentally combust your dry herbs. The good news is there are a few devoted people out there who are building DIY atomizers which CAN work with a 510 box mod, but there really isn't any sort of accurate temperature control or anything