The fundamental reason for air cargo inability to surpass passengers revenue is the

cost- because it is premium cost transportation medium than any other mode- the cost of transportation by rail, ship etc would in anycase cost less than air transport ,

aircraft are primarily designed to carry passengers, there has to be a compelling reason to ship by air- computer companies transport their hoods via air because the extra cost incurred while transporting via aircraft will be mire than set off by reaching the market earlier.

Thus, all of the above.