What is the etymology for Pinus halepensis?
If you search for this plant (properly spelled Pinus Halepensis) in the International Plant Names Index (IPNI), you'll find this page providing publication reference for this species of pine:

Gardeners Dictionary, Edition 8. London

According to IPNI, this publication by Phillip Miller in 17681 was the first to officially name this species. You can find the first half of he publication online here and the portion of the publication with info regarding P. halepensis here [p. 477].
Miller 1768 provides the following:


Pinus (Halepenſis) foliis geminis tenuiſſimis, conis obtuſis , ramis patulis. Tab. 208, Pine-tree with two narrow leaves in each ſheath, obtuſe cones, and ſpreading branches. Pinus Halepenſis, foliis tenuibus lætè viridibus. Rand. Hort. Chelf. Cat. 158. aleppo Pine with very narrow dark green leaves.


* Note: a longer description about this tree also continues on the subsequent page in Miller's book -- you can find it in the middle of the right column of the following page. 
However, this publication does not make clear why Miller chose this specific epithet.

Elwes &amp; Henry (1906)2 likewise describe this species [see online here] and mention that the epithet had been used for more than 100 years. However, they, too, do not provide meaning behind its use.



Further sleuthing suggests the following:
From Antropocene.it:

The specific epithet halepensis derives from Aleppo, the current city of Haleb in northern Syria.

Note, however, that this specific epithet doesn't only apply to this species of Pine. Linnaeus assigned this epithet to a grass (Sorghum halepense) he called Holcus halepensis (Linnaeus Sp. Pl. (1753) 1047)3 more than a decade before Miller named the pine in question.
Using this connection, SEINet suggests the following etymologies for halepense:

Halepense means &quot;from Aleppo, Syria.&quot;

This is the best I could do for you in a short amount of time. I, like you, could not find much explicit explanation for this word, and I admit I find its use a little strange given the relatively widespread range of the species mentioned (at least that both exist much wider than Aleppo).

 1. Miller, P. 1768. The Gardener's Dictionary, ed. 8. London. Pinus no. 8 
 2. Elwes, H.J. and Henry, A., 1906-1913. The Trees of Great Britain and Ireland. Edinburgh: R &amp; R Clark, 7. 
 3. Linnaeus, C. 1753. Species plantarum. https://doi.org/10.5962/bhl.title.669. [more info here and on wikipedia]