My guess is that its a just matter of fight of flight. There is not a lot of reason to wasting energy on immunity when you need to out run a bear who wants to eat you. Your body diverts resources to where they are needed most. 

Well, the immune suppression due to higher cortisol/corticosterone release only becomes significantly detrimental when the stress is chronic. In a short-term stress situation, as what probably would have been most situations in prehistoric man (and animals), the immune suppression is fleeting and probably not long-lasting enough to really have an effect on resistance to infection (and therefore survival).

It could however have significant benefits. The main mechanism of stress-induced immune suppression is decreased function of lymphocytes and other cells involved in the inflammatory and immune response. In a short-term stress situation, it could be useful to decrease inflammatory cell function, perhaps to save energy, but more importantly perhaps to decrease the potentially weakening effects of an on-going immune reaction (cytokine release from active immune cells will cause lethargy and weakness during infections). You would expect prehistoric man to frequently have on-going infections; a stress jolt would counteract the lethargy associated with infection and help to boost energy for a short while, which could be helpful in escaping the stressful situation.

This is just my 2 cents, i'm not a researcher in this field. Interesting question though.

I think this is a wonderful question. There is obviously some sort of evolutionary advantage, otherwise animals endowed with the "fight or flight" response would have vanished, vanquished by Darwinian pressures. My thought is that somehow "stress" in that setting is not the only difference- dietary and other environmental differences and perhaps even the nature of the threat/stress- chronic, with intermittent spikes - might dictate an advantage. 

Lombardi's answer is exactly what I think.

increase in cortisol levels in the body can act as an immunosuppression.

I agree with Erza's answer. Cortisols released during prolonged stress periods are immune suppressors. We can not estimate the stress level of the pre-historic man as compared to ours today, but we do know that the pre-historic man used to live a lot shorter than we do-though other non immune factors may contribute to a longer life too.