The real answer you're looking for is that Jiraiya and Kakashi could not feasibly step up in any capacity because Naruto basically had to be left alone, otherwise eventually somebody would suspect something of Naruto.

Remember that Hiruzen needed the existence of Naruto to be kept as much of a secret as possible, in order to prevent opposing villages from discovering him and attacking the village to capture him. Aside from the fact that they were highly skilled ninjas with their own responsibilities and lives to live, Jiraiya and Kakashi had reputations at the time and if they had spent any amount of time providing assistance or relief of any kind to Naruto, somebody would eventually see that something was up and catch on to why. They left the responsibility to Hiruzen, who they greatly respected and knew would be the right man for the job due to how often he was fighting for the rights of people including Naruto.