Why alcohol consuming had not been prohibited all at once?
One of the Gods names is Al-Hakeem (The Wise). That is to say, He creates everything in accordance with wisdom and usefulness. In fact, a humans growing up and being mature, a seeds leafing out and becoming a tree, an eggs cracking and a birds coming out of it all happens within a certain time limit. It is possible to see Gods law, which is implemented for universe in some of the religions orders. God, due to His name Al-Hakeem to eradicate the disease of alcohol habit from society, established the method of gradual prohibition. On the other hand, if alcohol would be banned all at once, people of that age who were addicted to alcohol would have feigned reluctance to convert (revert) to Islam. They would have not wanted to quit their habits. In this regard, related verses sent down gradually in the below listed order:
1. And there are (among the produce that God brings forth as nourishment for you on the revived earth) the fruits of the date-palm, and grapes: you derive from them intoxicants and good, wholesome nourishment. Surely in this there is a sign for people who reason and understand. (An-Nahl Surah, 16:67) In this verse, it is explained that intoxicants are not granted us as a means of subsistence. Some of the Companions of Prophet Muhammad quit alcohol consumption because it was comprehended that it was not religiously approved. Actually, as indicated in this verse, it was understood that alcohol was going to be religiously forbidden sometime in the future.
2. They ask you about intoxicating drinks and games of chance. Say: "In both there is great evil, though some use for people, but their evil is greater than their usefulness (Al-Baqarah Surah, 2:219)
3. O you who believe! Do not come forward to (stand in) the Prayer while you are in (any sort of) state of drunkenness until you know what you are saying, nor while you are in the state of ritual impurity (requiring the total ablution)... (An-Nisa Surah, 4:43) This verse forbids to pray (namaz; performing a ritual prayer five times a day) while intoxicated. In the view of circumstances, a companion who used to perform his namaz without any delay should not have consumed alcohol between two daytime prayers. If he would have done it, because of the state of drunkenness would not have been passed he would not be able to pray. He might have consumed after the nighttime prayer. However, great number of companions quit drinking alcohol, because alcohol addicted bodies were gradually keeping themselves away from alcohol.
4. O you who believe! Intoxicants, games of chance, sacrifices to (anything serving the function of) idols (and at places consecrated for offerings to other than God), and (the pagan practice of) divination by arrows (and similar practices) are a loathsome evil of Satans doing; so turn wholly away from it so that you may prosper (in both worlds). (Al-Maidah Surah, 5:90)
5. Satan only seeks to provoke enmity and hatred among you by means of intoxicants and games of chance, and to bar you from the remembrance of God and from Prayer. So, then, will you abstain? (Al-Maidah Surah, 5:91)
With these, last Quranic verses; it was precisely prohibited to consume alcoholic drinks. Narrated Anas bin Malik: I was serving Abu 'Ubaida, Abu Talha, and Ubai bin Ka'b with a drink prepared from ripe and unripe dates. Then somebody came to them and said, "Alcoholic drinks have been prohibited." (On hearing that) Abu Talha said, "Get up. O Anas, and pour (throw) it out! So I poured (threw) it out. Nobody will ever drink after that." (Translation of Sahih Bukhari Volume 7, Book 69, Number 488)