Why do prokaryotes need to be able to regulate their metabolism quickly?

The quicker the regulation, the less energy and material wasted to unneeded products and pathways.

Prokaryotes are organisms made up of single cell only. The cellular processes are not much complex because the energy input would increase then. Prokaryotes spend energy judiciously as the source of nutrition is very limited. The main aim is to reproduce or survive adverse conditions. That's why prokaryotes regulate the metabolism quickly.