At the most basic level it has to do with how the body synthesizes ATP, the anaerobic fermentation of ATP produces lactic acid as a by-product. During the absence of sufficient oxygen in the muscle tissue for purely aerobic ATP production the body has to resort to glycolysis to synthesize ATP to power muscle contraction.

Which leads us into muscle fiber types, since fast twitch fibers (Type IIB) fatigue the fastest due to factors such as worse capillarisation, ergo they aren't as easily supplied with oxygen for the aerobic synthesis of ATP. While the science says you cannot convert fast twitch to slow twitch fibres (Type I) you can modify fast twitch fibers through endurance training to intermediate fibers (Type IIA), which have improved fatigue resistance due to factors such as improved capillarisation.

The muscle fibre composition of your body is currently thought to be dependent on genetics, and composition varies from muscle to muscle throughout the body. For example, the superficial Gastrocnemius (calf) muscle typically has a high composition of Type IIB fibers while the deeper Soleus (calf) muscle is typically composed of Type I fibres, the former suited to explosive movements and the later to endurance activities.

Additionally, post-exercise about 80% of lactic acid enters the bloodstream and is reconverted to pyruvic acid by the kidneys, cardiac muscle and liver via oxidization. However, the oxygen debt accrued during exercise must be "repaid" in a number of different areas; for example, elevated body temperature increases the metabolic rate and rapid metabolism consumes extra oxygen.