Why not Preterism?
To respond to this question-Context is the key.
Since the "Drying up of the River Euphrates"(Rev. 16:2) occurs as the 6th vial(bowl judgment) is poured out, which was only after the Lamb was given the ability to open the book and loose the seals(Rev. 5), which only happens after John is sent to Patmos(92-95AD) which is significantly later than the Destruction of the Temple-which Preterism says ushers in the eternal age of the Kingdom
Adam Clarke [1762-1832] comments on verse 30: “The plain meaning of
this is, that the destruction of Jerusalem will be such a remarkable
instance of Divine vengeance, such a signal manifestation of Christ’s
power and glory, that all the Jewish tribes shall mourn, and many
will, in consequence of this manifestation of God, be led to
acknowledge Christ and his religion.”(Taken from here)
Although Revelations does look backwards(Chapter 12), there can be no case made for the bowl judgments preceding the seal and trumpet judgments, which of their nature are incremental and proceed from Chapt. 5, therefore to follow the "70AD" argument is ignoring any context of the judgments to make them 'fit' a Preterist scenario.
Another contextual point to be made is "Why is the Euphrates River is drying up?" We are told in Rev. 16:12-14,
And the sixth angel poured out his vial upon the great river
Euphrates; and the water thereof was dried up, that the way of the
kings of the east might be prepared.
13 And I saw three unclean spirits like frogs come out of the mouth of
the dragon, and out of the mouth of the beast, and out of the mouth of
the false prophet.
14 For they are the spirits of devils, working miracles, which go
forth unto the kings of the earth and of the whole world, to gather
them to the battle of that great day of God Almighty
There was no advance on Rome during the time of Jerusalem's Destruction from the east, indeed, it came from the west. Rome had advanced to the Euphrates River at the height of their empire(117AD-taken from here)
So, if it didn't occur during the 1st century, then what is the significance of the "Drying of the River Euphrates" and what time frame must we approach as to it's fulfillment. For the sake of brevity I will take a Historicist position, although it may not be one easily recognized.
Context is Key
In order to examine in detail the scope of Rev. 16:12-14, one 1st has to ask, is the context Literal or Figurative? This is NOT the same question as "Real or Imaginary", or "Literal or Metaphorical". We are asking if the language in it's historical/ grammatical sense actually communicates the meaning, or is it representative of another reality.
The Euphrates River is the boundary of the land God promised to the descendants of Abraham (Gen. 15:18),
In the same day the LORD made a covenant with Abram, saying, Unto thy
seed have I given this land, from the river of Egypt unto the great
river, the river Euphrates:
therefore, the River Euphrates must be considered as a border to this Covenant land. Therefore, whoever trespasses this boundary is seen as one doing harm to the nation of Israel. But before one proceeds to a "Literal" interpretation of this event, lets look at the rest of the passage in context:
1) Water drying up
If the context is telling us of a future event, the "drying up of water" poses no significant challenge for a modern army, nor can be seen as one engaging the entire earth. The Euphrates was 'forded' by previous empires making their conquest, but was not seen as any major obstacle; therefore, the understanding of what "drying of the Euphrates" must be representative.
2) Three spirits like frogs out of the mouth of the dragon, the beast, and the false prophet. Since 'frogs' are representative of unclean animals, the dragon is representative of Satan(Rev. 20:2), the beast and the false prophet are also representative(Rev. 13:1/11) therefore the context also must be understood figuratively.
The Futurist sees these events has happening in the future; we are well into the future from when Revelations was given. Dispensationalists separate Israel and the Church from this present age, therefore in Dispensational eschatology this event can only happen after the Church is "Raptured" and Israel is by herself to face all the hostile nations against her. The fact is Israel is NOW facing that reality as a nation, and never in history AS A NATION could she have faced the reality of every nation of the earth against her. So we are currently living in the climate where Israel could be attacked by every nation on the earth opposing her; we see the UN Resolution 242 committing Israel to return all lands gained in the '67 war, including Jerusalem-every nation was signatory to this document. Therefore all nations have committed themselves to the referendum following it, the details yet to be worked out.
But why the "water drying out"?
The water here is of special significance; it doesn't say that "God dried the water", or that "a dam was built"(there are numerous dams, they benefit everyone in the region) Though there is no scriptural precedent for this, the water is in fact oil. What brings all the nations of the earth to the battle of Armageddon is not the Arab water, but the Arab oil; in 1973 the Arabs decided to "dry up" the wells of Saudi Arabia and other Arab entities, and the shortages in this country and many others led us to 'compromise', forcing Israel to 'accept' trading land for peace. This situation has not abated, the Western(and eastern) nations are in just as much need of Arab oil now as before. The fact that the "Euphrates is crossed" indicates the hostile intent of the Arab nations, who collectively hate Israel and see the flag of their allegiance fly over Jerusalem; the 'water drying up' is the means by which all the nations of the earth get involved-it is vital to their national interests that a solution is rendered.