Riot was trying to be less definitive about who Should be able to Jungle. When they made the Jungling passive more transparent for different kinds of Junglers. Though they definitely don't want Certain champs Jungling (Like Alistar)
AP Jungler need to be Extremely kit reliant. They usually lack the sustained damage from Auto Attacks that ADs have. And they lack the Tanky sustain that most Tanks have. Yet despite that, they can't have lots of overt damage for Clear speed, As that can make them Abusive in lane. But the Item can't be OP either, for pretty much the same reason, because they don't want it abused in lane. So to that end, not any AP champion can Jungle. It has to be someone who can jungle decently well, even before getting a Jungle item. (You could add "bonus damage to monsters," to some of their abilities, but that's pretty lazy.)
Over all with Jungle item balance. Clear speed, is the primary metric for measuring the balance of Jungle items. This is due to the fact that if one class of champion had way better clear speed that the other, the others would be perpetually behind in gold, level, and experience. (which isn't a good reason for an item/class of champ, to be strong). This was most noticeable with the introduction of Cinderhulk. Before Cinderhulk, Tank Jungler were did not have much in the way of clear speed. Which not only put them behind in gold/exp, but made them Vulnerable to invades. And Juggernaut was the only Jungle item who did not help their respective Junglers clear in some fashion. (unless the Tank Jungler had HP ratios, and most of the time those were moot) With Cinderhulk Tank Jungler's Clear speeds are more comparable to their counterparts, while giving them a scaling late game advantage as a counterpart to the early advantage that warrior has.
Last point I'll make. I don't like the Idea of giving more sustain. (any more sustain on a jungle item that the jungler passive + smite buffs) It would give them a Unique Advantage that other Jungler won't have. (A possibly Oppressive Advantage) And that is, having enough sustain to return to the jungle after a gank (success or failed) and come out with enough health to perform another gank. The only way to match that is if you have the Sustain in your champions kit. (Nid, yi, anyone with a heal.) Or if you buy Life steal. And all that said, that advantage falls off if your going glass cannon AP. You'll still be just as easy to kill unless you had the defenses or the dueling power to make use of the spell vamp to full effect. So Even that would sub divide viability (or competitiveness) of AP Junglers.
You could add sustain on AP Junglers on a champ to champ basis where they make sense for their kit. I think it would be a more interesting mechanic to avoid damage, all together. Either by CC the camps so they can't reach you, or using a pet to soak up some of the damage, while increasing your DPS. That is to say, If their are any non Jungling AP champs who are good candidates to become Junglers, It would be the likes of Zyra or Heimerdinger.