I think you should perhaps clarify what you mean by 'why'.
Why from a rules perspective: Because that it what it says it does.
Why from a balance/game design perspective: As Quixote noted.
Why from an 'in world' perspective: Perhaps while the magic makes you feel super hungry and makes eating difficult, resulting in weakness, it isn't powerful enough to inflict lethal damage.
Quixote wrote:Scorching ray is a level 2 spell and it can kill you (and doesn't take days to it.)
Because it's a level 2 spell?
Because it seems more about making you suffer?
...because it's a level 2 spell?
cure light wounds (1st level spell) can heal the damage caused by a scorching ray. It can not heal the damage caused by feast of ashes.
Characters who have taken nonlethal damage from lack of food or water are fatigued. Nonlethal damage from thirst or starvation cannot be recovered until the character gets food or water, as needed—not even magic that restores hit points heals this damage.
Obviously no one is saying it is unbalanced for a 2nd level spell to deal lethal damage.
Feast of Ashes provides a long term negative effect that is difficult to remove when it could first be encountered. The balance to that is, that while it takes at least a 3rd level spell to remove, and the effects are fairly bad, it won't actually kill you.
This also makes it easier for a GM to use against a PC without it being just a death sentence. Personally, I think Feast of Ashes designed at it is makes it far more likely to actually be used in a campaign then if it eventually did lethal damage as well.
Obviously no one other then the designer probably knows why the particular design choices in the spell were made, but those are some good reasons. I am sure others are plausible as well.
Perhaps you can think of some on your own if none of the presented thoughts are satisfying to you.
Feast of Ashes is a "flavor" spell that is inherently designed to cause long-term suffering. This is exclusively for Druids and Witches, similar to the Cup of Dust spell (except Cup of Dust *can* kill you), and sometimes if you piss off the Druid/Witch by cutting down a bunch of trees or mushrooms or w/e the Druid/Witch loves, they want to curse you with a torturous spell via Feast of Ashes and Cup of Dust so that IF you survive, you can tell the story to others so that it never happens again.
This is actually a great story-hook. PC's arrive in a town and head to the local watering hole for food, ale, and information. An hour after the PC's arrival, an NPC bursts in the door frantically gasping for food and water, though nothing slakes his hunger or thirst. "I've been cursed by Druids!", the man pleads for help, dry-heave vomiting what seems to be sand and ash. If you choose to help him and remove the curses, he tells you that he chopped down a tree for firewood at his cottage. Further investigation reveals that almost everyone living in cottages in the woods have been slain or cursed by vengeful Druids.
Scorching ray is a level 2 spell and it can kill you.
Scorching Ray does 4d6 damage. Full stop. It can kill you, if you have 24hp or less until you hit -Con.
It's just like the ridiculous argument with Aboleth Lung (albiet much less extreme); save-or-die does not exist at level 2, even if it's save-or-die in several days.
Let's change the question:
If you want to kill your target, why are you using Feast of ashes?
There are plenty of level 2 spells that do lethal damage.
If the goal is to kill the target you can use them.
If the goal is to incapacitate a target to get an easy kill you have another large set of spells.
Then you have Feast of ashes. Its goal isn't to kill an opponent, it is to make it suffer.