There are a number of behind-the-scenes reasons like what they've evaluated the risk of worker injury to be, what their most common equipment (aircraft holds, containers, baggage handling at airports, etc.) can accommodate without risk or the leftover of previous competitive decisions -- e.g. for a long time "every" airline allowed 2 free checked bags because that's what every other airline allowed and over time various airlines figured out different ways of profiting -- either from charging to check luggage or by marketing the "free" bags and that has also driven changes in where airlines place a cutoff for the maximum sizes and weights.

In any event, at least with United -- since this is the United subforum -- some oversize items, particularly specific pieces of sporting equipment, are acceptable "if aircraft size and load conditions permit" (and in many cases excluding itineraries with United Express flights) with greater maximum dimensions than the standard checked baggage limitations. These include javelins, hockey/lacrosse sticks and curling brooms/brushes, hang gliding equipment, pool cues, kiteboards, surfboards, wave skis, windsurfing board/mast/sail, and a few others, so you may have options depending on what the large item is.