The reason, fundamentally, is the way jumps are calculated: the total ship mass is set against the FSD class and size, combined with the distance of a jump, to determine the fuel consumption. Every FSD then has a maximum fuel consumption per jump, which determines the limit on the jump range.
For speed, agility, shields or armour, the ships have built-in base values and the equipment adds modifiers to these values. For hyperspace jump fuel consumption (and therefore jump range), no such built-in values exist; only the above consideration is necessary to calculate the result*. Therefore, some ships have unusually high or low hull mass considering their size - this is how FD can adjust the jump range of a ship. Apparently FD decreed the DBE must have a smaller jump range than the Asp, and therefore set its hull mass accordingly high. It's also why the Courier has such a low hull mass (possible lore reasons "Gutamaya uses ultra-light-weight high tech materials" aside), and the Dropship almost 50% more than an Anaconda.
*In fact this is a good thing, and makes lots ofs sense from a perspective of the fictional science of the FSD; the problem is rather the coarse scaling of FSD classes. If there were, for example, an FSD class between 4 and 5, the DBE might have ended up lighter but with that intermediate FSD size, to satisfy the condition "jump range Asp > DBE".
Since speed and agility scale off built-in values specific to each ship, that means it is meant to be this slow, because they could increase the hidden values to compensate for the mass of the ship if it were meant to be faster.