Actually, mulching is mostly about preventing soil erosion and increasing water retention. When you consider that the original reason for abandoning the orchards is expensive water you may think that burning your orchard residue is setting money on fire.

It is the same with tillage. Conventional farming sprays synthetic fertilizers, only to do tillage which resets the soil and basically throws all the money they put into their fertilizers away, necessitating even more fertilizer usage.

> Would it not make sense to let the crops decompose into organic matter for future crops?

Orchards don't die and decompose in one year. Presumably they want to put the land to productive use next year.

You can do that without burning, it just costs more money which is why they prefer burning and polluting local air until regulations inevitably catch up and outlaw the practice for good in the future. Just because its cheaper to do things like burn your waste or dump it into the rivers and oceans than to do something more sustainable, doesn't make it a great idea on the whole.