The 1:1 Scale Weathering Project in Progress at Jacumba - [2015 Summer Solstice +7 Series] -A Tren Turistico Del Noroeste (Tourist Train of the Northwest) Caboose at Jacumba "weathers" delicately under the Sonoran Desert sun because that is what happens to persons and equipment when they are left exposed to the elements.
As to the "why" is this happening (the 1:1 weathering project) who knows for sure, but were I to speculate (because I am just pulling this out of thin air so to speak) I would say these items are out here because the people that own them cannot afford to move them somewhere else.
Further, the lessor of the track through the yard (the Pacific and Imperial Railroad - PIR) is also unable to move the slowly decaying rolling stock in the picture because there is a derailment of Metra cars roughly two miles down the line towards the gorge at Dubbers Spur, and a number of rock falls that block eastward movement on the Desert Line between Tunnel #5 and Tunnel #21 near Dos Cabezas.
Moreover, since there is no working agreement between the PIR and the operators of the track in Mexico, the westward approach to Jacumba is also blocked by the international border and all the politics associated with the movement of international commerce.
Consequently, these items will continue to remain where they are located at Jacumba for the foreseeable future.