A greedy approach (always picking what looks best right now) says: "always pick the smallest available element to leave room for more." But this fails. Consider [3, 1, 2, 4]. If you start with 1 (smallest), you get [1, 2, 4] with length 3.
But what if the array were [3, 10, 2, 11]? Starting with 2 gives [2, 11] with length 2. Starting with 3 gives [3, 10, 11] with length 3. The problem: you don't know which starting point leads to the longest sequence. A small element early might not connect to later elements. You need to explore all possibilities.