Because airlines are generally deregulated on most elements of their “terms of carriage” (technical jargon for what they can charge, when they must provide compensation etc.) . There are very specific areas where the law or FAA rules dictate compensation such as denied boarding for overbooking, keeping passengers on the tarmac too long, etc. but “mechanical delays” are not included since there is no way to define the term and not wanting to create disincentives for airline employees to flag and fix problems. Indeed, flying with an even minor mechanical issue that has some safety component can result in fines in the hundreds of thousands of dollars. Instead, the current philosophy is to let the market regulate on-time performance and delays and the airlines must publish performance data so you can choose a flight based on the livelihood of non-weather delays.
