Candymaking is extremely sensitive to temperature. If the mixture heats higher than the point that produces the desired texture, you're basically out of luck. That makes it very critical that you reach and not exceed your target temperature.
This is particularly difficult when making candy because much of it starts as a sugar-and-water syrup. Water has an especially high heat capacity, meaning that it takes a relatively high amount of applied heat to raise its temperature. This is why a big pot of water takes a long time to boil. It also implies (because diffusivity is calculated from heat capacity and density) that it takes a while for applied heat to diffuse through the entire pot; that is, the bottom of the pot where heat is being applied will be significantly warmer than the top, absent more active circulation like stirring or the pumping action of an immersion circulator.
Anyway, "boiling" implies greater heat application than "simmering". The more heat you're applying to the bottom of your pot, the greater the difference between the top and bottom. If you're then measuring the temperature at or near the top, you may be getting a wildly inaccurate measurement of the "total" heat of your syrup. Once the heat diffuses completely, the mixture will be several degrees warmer than you measured, and that can completely ruin an entire batch of otherwise-delicious fudge.
Simmering is recommended because it gives the heat being applied to your syrup more time to diffuse, which means more even and consistent heating, which means you can reliably hit your target temperature without going over. If you became some kind of ninth-dan fudge guru, maybe you could boil and know exactly how much heat is enough to hit your target, but don't count on getting there.
The sensitivity of this process is part of the reason that candymaking is considered one of those semi-advanced cooking topics. If it was easy, we'd probably all overdose on fudge by the time we learned to safely operate a stove.