Why haven't prey evolved the ability to always outrun their predators?
There are (at least) three important factors to consider here; evolution under selection requires genetic variation upon which to act, selection can act on covarying traits causing trade-offs, and adaptation also occurs in the predator. A lot of this is covered elsewhere on this site (including the effects of the other mechanisms of evolution), but little specific reference is made to predator-prey co-evolution.

Adaptation and genetic variance

One of the mechanisms of evolution is selection, and adaptation occurs when species evolve as a response to selection. For a response to selection to occur there must be genetic variation within the trait, such that the genes an individual carries affect the individuals fitness. For example, there may some genes which give the carrier better muscle structure for fast running, and this increases survival which, in turn, increases reproductive output. The importance of genetic variation is often overlooked, but is highlighted by the breeders equation. Genetic variance can arise by novel mutations, or can exist as standing genetic variance.

Selection on covarying traits

Selection is seldom, if ever, a univariate process. That means fitness is not determined by a single characteristic, so fitness in a prey species may be determined by the speed at which it can run, but also its metabolic rate, stamina, ability to acquire nutrients, the way it provisions nutrients for growth and repair etc.. Genetic covariance between traits (induced by linkage or pleiotropy) can impact the response to selection, because any selection acting on a covarying trait can cause affect the adaptation occurring in the focal trait (strengthening, weakening, neutralising, or even reversing the direction of response). Therefore, if running speed covaries with other traits it may be difficult for selection to increase running speed. Think about athletes, I doubt Usain Bolt could run 10,000 metres as fast as Mo Farah and vice versa, because there is a trade-off between speed and stamina.

Adaptation in other species

If selection did cause speed to increase in the prey species then that would strengthen selection for increased speed in the predator. This is called an evolutionary arms race, where the adaptive the evolution of two species interacts causing adaptation and counter-adaptation. For example, genes which allow faster running might spread through a population of gazelles (prey) because carriers are more likely to outrun the lion (or the other gazelles), but this will increase selection on lions (predator) to increase speed (or adopt other strategies) which will spread genes to make the lions faster (use other strategies); the result being adaptation and counter-adaptation between the gazelle and lion populations. 



There is a little series here about Butch Brodie and his studies of coevolutionary arms races between garter snakes and toxic newts. It would be good to read on the Red Queen Hypothesis, which describes how species, under interaction with other species, must continue to evolve just to prevent extinction. Also worth noting that prey species often have more than one predator species (and predator species often have more than one prey); interaction and coevolutionary networks are complex. What may be adaptive in some interactions may also be maladaptive for other interactions (see specialist and generalist, and selection on covarying traits).