Understanding an apparent conflict regarding Israel's Biblical eastern boundary

Question

I'm trying to understand what appears (for me) to be a conflict regarding the Biblical eastern boundary of the "Promised land".

It seems that the land promised to the fathers went all the way as east as the Euphrates. There are two verses, in particular I'm viewing:

Breishit 15:18 G-d promises to Avraham and his descendants the land from the Egyptian river to the Euphrates river.

Devarim 1:7-8 In verse 7, Moshe tells B'nai Israel to go to the land until the Euphrates River. In verse 8, Moshe uses the phrase "...that G-d has promised your fathers."

What seems contradictory is that when delineating the land borders for 9 1/2 tribes (Levi doesn't get a portion; and 2 1/2 tribes took their portion East of the Jordan) the Torah marks the eastern border along the Kinneret and the Jordan River - not as far as the Euphrates (Bemidbar 34:10-12.)

The conflict appears to be why G-d limited the eastern border for those remaining tribes to the west of the Jordan. In actuality, it seems that the "promised land"'s eastern border extended to the Euphrates. Why, then, does the Torah in Bemidbar not extend the eastern border for all the tribes to go east of the Jordan?

Answer 1

Bereishis Rabbah 44:23 notes that Avraham was promised 10 nations' territory in the succeeding Pesukim in Bereishis 15, but his children only got 7 nations' territory. The Midrash simply identifies the three nations left out: Arabia, Shalami, and Nabatea; or Damascus, Asia Minor, and Syria; or Asia Minor, Turkey, and Carthage; or Edom, Moav, and Ammon. The Midrash also notes that we will get their territory in the future:

ולמה נתן להם שבעה? פירשן לעתיד לבא

And why did He [only] give them seven? [Because] he designated [the other three] for the future to come.

(Translation and emendation of Midrash text based on Artscroll, in turn based on Eitz Yosef.)

The Eitz Yosef also points out that according to the first set of three, the Bnei Yisrael indeed did get their ten nations' worth, all the way to the Euphrates, in the days of Shlomo HaMelech, but only for a very brief period of time.

The Midrash concludes that in the future, the nations presently occupying these lands, whoever they may be, will be exiled (as per Maharzu and Eitz Yosef) and we will take those lands for ourselves, thus fulfilling Hashem's original word. But for the time being, we would only have the land belonging to seven.

The question still stands: why is this the case? According to the last opinion, this makes sense: we are commanded in this world not to provoke Edom, Moav, and Amon (Devarim 2:5, 9, and 19, respectively). But according to the other opinions, why did we not merit them in this world (permanently)? Who knows. The Midrash and commentaries don't explain.