If you overcook a steak it will end up tough and hard to chew. If you cook ox tail for a few hours, however, it will become extremely tender.
What determines whether meat becomes tough or tender when cooking it for a long time? How can I maximize tenderness?
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.
The answer by @moscafj is spot on - the main contributing factor to this difference is the connective tissue, this is the difference in those slow-cooked pieces of meat that make them fall apart, fork-tender. The connective tissue breaks down into gelatin which provides loads of lovely moisture and lubrication to the meat, and that lovely soft experience we all recognise with slow cooked meat.
What is connective tissue though? It's the part of the muscle that basically holds stuff together (broadly speaking) - so the more strain the muscle is put under, the higher the amount of connective tissue you can expect. This is why you might hear people refer to certain cuts of meat as "hard-working" - this means the muscle gets a lot of use, so it has lots of connective tissue, and will be a good cut to slow cook.
Great examples of this would be beef/ox-cheek (think about how much a cow uses its cheeks to chew all day long! cheek is one of my favourite cuts of beef for this reason), ox tail, brisket (brisket can be responsible for supporting about 60% of the cows weight)
This also isn't limited to beef/pork, but the same factor can be seen in chicken breasts vs thighs/legs - chicken breast muscle doesn't do much day-to-day, so is lean with limited connective tissue, and dries out quickly as you cook higher temps, where as the thighs and legs get constant use, so they can with stand much higher temps of cooking and still be deliciously juicy (also why I prefer cooking thighs!).
