As <PERSON> points out, the ratio of torque to tension is lower the lower the lead angle is. Since the important measure of bolt tightness is generally the tension in the bolt, we want to achieve that minimum pretension with the least effort possible. Assuming we have to maintain a certain shear area of the thread (so the the threads are stronger than the bolt when fully engaged) having two starts means we double the lead angle and greatly increase the amount of force required in the wrench to tighten the fastener appropriately. On its own though, this isn't the end of the world in practical applications because a big enough torque arm, (or a shear wrench) makes this just a matter of using a bigger motor.
The bigger problem is that we want bolt threads to be self-locking. That is, we wouldn't want the pretension in the bolt to cause it to loosen. 333
Imagine if the bolt had 10 starts, and therefore a very steep helix for the threads - no matter how hard we tighten the bolt, the pretension will immediately loosen the bolt when we let go of the wrench. This is because the lead angle lets so much force transfer into the rotation of the bolt (or nut) that it can overcome the friction between the internal and external threads. This would make the bolt not very effective without an external locking device. By contrast, standard single start fasteners are often pretensioned (or simply made snug tight) and trusted to self-lock based on their shallow lead angle. In situations with high vibration or thermal cycles, additional locking elements may be used, but they aren't normally required if the bolt is properly tightened.
This is why single start threads are typically used for fasteners (things that aren't supposed to move) but multiple-start threads aren't uncommon for leadscrews (which are supposed to move freely, or have an external brake.)
Screws which form their own mating thread in another material are often two-start, as they have an additional resistance to unscrewing from the compression and roughness of the substrate they displaced around the threads. This is the case with some sheet metal screws, and also most wood screws. 333