Why is the Holy Land holy to Muslims?

I know why Jerusalem is, but the Holy Land is the whole area between the Mediterranean Sea and the Jordan River. Why is this whole area considered holy?

Because Jerusalem was the prayer facing direction before mecca and all Judaic prophets are in Islam and, for instance, the angel Gabriel spoke to Muhammad.
Meaning; Islam is a branch from Judaism in much the same light as Christianity is.
Many Muslims, for instance, consider Christianity a sister religion (in fact allowing some old Christian followings to remain in their societies, and even in “royal” advisory positions at times), but simply, as the Mormons put it, not practicing or following the fully revealed truth.

Why is any land “Holy” to anybody?

Yes, but that accounts only for Jerusalem (except, you said all Judaic prophets, I don’t know where they come from), but the whole area from the Mediterranean to the Jordan River is considered holy.

Or are you saying that it is holy to them for the same reasons it is to the Jews? If so, could you explain why the Jews consider it holy (the whole land, aside from Jerusalem).

Which description are you looking for here?
Migratory, or theological?
Since it’s the shorter of the two options, I’ll take migratory first here as an addition.
The answer to why is consequence of location.
This land is not directly Egypt, nor any other yet fully established giant empire.
There were other civilizations, but all intermixed who had what when in that same region; mostly without much play from Egypt.
This land, for any peoples living in this region is a sweet spot of life.
There is a very large body of water with a flowing river to fish and build cities and create farms and mining on one side.
On the other is access to the primary trade waters of the “world”.

This is the good land of the area.
If you want a clean answer, much akin to Trajicomic suggested, this is your answer.
The land is holy because it is life-land.

I’m looking for a theological explanation.

Maybe you’re being too narrow in your thinking. Recall, the whole earth was promised to us, not just the specific region you are referring to. It’s all holy ground. Or could/should be.

To be specific, I have to write a paper about the crusades and I have a paragraph where I explain why the land was considered Holy by the Muslims. While there may be other interpretations of why this land was so important (as described by Jayson), I’m looking specifically for the theological interpretation.

As I said, I have the explanation for Jerusalem, but why is this whole area from the Mediterranean Sea to the Jordan River considered holy.

Again, there may be other, more cynical interpretations as to why the Muslims wanted this land very badly, but I’m looking specifically for the explanation of why it was considered holy.

God promised the land to Abraham’s descendents and, as Jayson noted, Muslims claim to descend from Abraham through his firstborn (though illegitimate) son Ishmael.

Thus the land promised to Abraham’s descendents is the land promised to Muslims.

I’m no expert though.

Nope. That’s the theological variant summarized nicely.