I'm guessing that it is the reduced cooking time--though certainly meat quality can be a factor. I'm basing my guess on my experience, my knowledge of food chemistry and my understanding of how the tastebuds/brain act/react to various food/taste stimuli. I'm not going to say I know the answer to your question, but based on these issues, and on the experience over many years of cooking the progressively leaner meats that have been, unfortunately, demanded by American consumers, that's my response. Perhaps I'm wrong. It's harder to wager a guess (or to 'know') having not been present when you cooked the brisket you described.
Even today, though, one uses a different approach in terms of temps when one cooks Prime brisket vs. Select, or when one cooks supermarket ribs vs. ribs from a pastured Berkshire pig. At least I do anyway. The better marbled Prime and fattier Berk ribs call for lower/slower cooking than does the Select or the market ribs.
Another variable might be at play. Normally I use the term 'caramelization' pretty loosely; I'm not going to here.
Caramelization, the chemical reactions that occur when sugar is heated to the point that its molecules break apart, creates hundreds of new flavor compounds and causes the browning we associate with the word.
Maillard reactions, similar though more complex, that also cause browning, occur when heat is applied to a carbohydrate (free sugar) molecule with the presence of protein or an amino acid. Maillard reactions are what are primarily responsible for the color and flavors of roasted, grilled and sauteed meats, bread crusts, unsweetened chocolate--items that are not mostly sugar, though, in the case of rubbed meats or, for another example, doughs (like cookies) that have a large sugar component, caramelization can also occur, depending on the cook temps. Caramelization begins to occur at temps of 330 and above; Maillard reactions start lower, somewhere between 230 and 250.
Maillard browning reactions, especially, due to their complexity and the vast number of flavor compounds created during the process, are largely responsible for how we react to foods that have undergone this process. One can cook a steak, for example, or, say, a dinner roll, in such a way that it is cooked to 'done' but not browned. The difference, though obvious, is pretty remarkable, and our taste buds (and brain) react much differently when tasting the two. The complexity and number of the flavors developed due to Maillard reactions cause increased 'tastiness', increased salivation, and an increased perception of 'full-flavoredness' because of how our tastebuds/brain reacts to this type of browning. With steaks, for example, it can make them seem more moist, even though they in fact have lost a bit more moisture at the surface due to being seared.