What was the reason for the suspension of the “vegetarian mandate” in Gen 9:1-3 after maintaining it for over 1500 years?

Issue 1

One reason God might withdraw the vegetarian mandate for Noah is that Noah was the savior of the animals by bringing them onto the ark. His dominion over them was more absolute than Adam's, because without him they would not exist.

A second, more likely, possibility is that God withdrew the mandate as a response to Noah's offering, made immediately prior to the mandate's suspension. This is strongly hinted at in the text:

Noah built an altar to the Lord, and took of every clean animal and of every clean bird, and offered burnt offerings on the altar. And when the Lord smelled the pleasing odor, the Lord said in his heart, “I will never again curse the ground because of man. (Gen. 8:20-21)

Thus, God may have been so moved by Noah's offering with its odor of cooked meat, that He wished to share His pleasure with Noah and his descendants. Perhaps more important, this sacrifice was the culmination of Noah's long course of righteous suffering, so we can presume that God was not only pleased with the aroma of the meat but also with the piety and heart with which it was offered. In the same verse, God pledges never to curse the earth again. Immediately afterward He blesses Noah with the removal of the vegetarian mandate.

Issue 2

Removing the vegetarian mandate reduced violence by reducing hunger. During the period between Adam and Noah, there were certainly periods of drought and famine, just as there were during the time of Abraham a few biblical generations later. This would lead to wars over land and was probably a contributing factor to the violence mentioned in Gen. 6. Animal husbandry would have ameliorated both starvation and violence. Thus, if we take literally the idea that God repented because of the violence, God may have taken partial responsibility for the conditions that led to it. Thus, he removed the vegetarian mandate after the Flood.

Issue 3

The vision of the peaceable kingdom in Isaiah supposes a time when humanity returns to the Edenic state. It is debatable whether this is meant literally or in a poetic, hyperbolic sense. For my personal answer see the addendum below:

Addendum: was this "mandate" real?

The supposed "vegetarian mandate" is not actually specified in the Bible. There is no "thou shalt not eat meat" in the text as there is with regard to the fruit of the Tree of Knowledge. There is simply a statement that fruit and vegetables are permitted. The same permission is given to the animals.

And to every beast of the earth, and to every bird of the air, and to everything that creeps on the earth, everything that has the breath of life, I have given every green plant for food. (Gen. 1:30)

But are we to suppose that predatory animals were vegetarians both prior to Adam and between the time of Adam and the time of Noah, or that lions, cheetahs, and raptors were not predatory? Perhaps some do, but IMO the vegetarian "mandate" reads something into the text that is not there. It may be argued that the animals also fell when Adam did, and therefore did not follow the supposed vegetarian mandate that God gave them, but this too is not in the text. It also contradicts God's declaration of responsibility for "natural evil" in the Book of Job:

Is it by your wisdom that the hawk soars,
    and spreads his wings toward the south?
 Is it at your command that the eagle mounts up
    and makes his nest on high?
 On the rock he dwells and makes his home
    in the fastness of the rocky crag.
 Thence he spies out the prey;
    his eyes behold it afar off.
His young ones suck up blood;
    and where the slain are, there is he.” (39:26-30)

Among the ancient Rabbis, some did accept the idea of a vegetarian mandate, but others did not. Working back from the eternal laws revealed at Sinai, Rabb Yaacov of Chanin said:

“When will it become fit for eating? When it is slaughtered. Here is hinted the [later] prohibition against eating a limb from a living animal.” (Bereishit Rabbah 16:6)

In other words, eating meat with the blood still in it was always prohibited, both to Adam and to Noah. And both of them were also allowed to eat meat that was properly butchered and cooked. This rabbinic interpretation is just as sensible as the theory of a vegetarian mandate.

This is not really explained. I would go along with your analysis in that in Genesis 1-2, animals are not food. How could they, if there is no death yet? But as soon as we leave the garden, the first things that happen are:

1. God makes clothes from animal skin, teaching man by example that animals are also resources
2. Abel herds animals, and sacrifices them. This is compared to Cain who grows plants (food!). Abel is the righteous one.

While I don't see it is made explicit that animals were eaten, both clearly establish that man has some kind of authority to kill animals. I think it is not far reached to assume they were eaten (after all, later priests also ate parts of the sacrificed animal, and clearly they were used for resources, and it would be a waste to not eat what you killed for clothing). But no explanation whatsoever. Sacrifices that God approved, but no recorded commandment to do so. Something happened here that is, quite simply, not recorded. As such, I challenge that a vegetarian mandate was actually in effect after the garden was left. What Noah was told mirrors what Adam was told in the garden, minus the eating animals. Speculation: Maybe it actually mirrors, in full, what Adam was told after he left the garden?

Genesis 4 essentially just says: after the garden, animals are resources and can be killed, but man shall not be killed. And immediately after comes Noah. No time to explain, we want to get to Abraham ASAP, apparently ;-)