Quite simply, active transport must be used to transport sugars and other solutes into the phloem because no other biological process can do it. Biological organisms are lazy; they don't want to spend energy if they don't have to. Active transport requires energy, so organisms only use it when they must. In the case of sugars, they are too large to cross cell membranes, into or out of the phloem, so they must go through specific transporters, which are transmembrane proteins. It requires energy for this to occur, thus we call it active transport.