Because SE wants to limit the liquidity of reputation. Instantaneous awards would make it possible to transfer an arbitrary amount of reputation to multiple users in a short period of time. This has some potential to be useful, and also the potential to be disruptive. For example, consider the bounty spree of Frictionless Jellyfish (discussed on meta.MO and tea.MO) -- it is at present limited by the inability to give more bounties before awarding the current ones, which cannot be done before 24 hours elapse.
It is not very hard* to imagine a high rep user getting offended or otherwise disenchanted with the site, and making odd decisions, such as giving 3000 points to a dozen of the worst-behaved users on the site. So, I think that the potential disruptions of instant bounties outweigh the potential usefulness.
(*) Especially for me.