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!).