What is the most analytical theory available for why recipes will taste good, such that they could be designed ahead of time and predicted to be tasty, and explained why? 
Fat, collagen or connective tissue, and cooking time and temp, all have an impact on tenderness.  Steaks with uniform fat marbling are graded higher because, not only does fat add flavor, but when it breaks down it makes the steak more tender.  Collagen begins to melt and dissolve to gelatin between 160F and 180F (71C and 82C). Oxtail (in your example) has much more fat and collagen than your steak.  When the fat and collagen break down, you get tender and moist meat.  Since steak has some fat, but much less collagen, it will overcook, the fat will melt out, and you will be left with an overcooked and dry product. Of course well-cooked steak can be tender (though, depending on the cut, some more tender than others)...and poorly cooked oxtail can be tough, so applying proper cooking technique to the particular cut of meat to maximize tenderness is critical. If you want maximum tenderness, find a high fat, high collagen cut, and cook it with moist consistent heat until the collagen melts out....short rib, or pot roast come to mind as examples, along with your oxtail. 