Jesus reveals special truth to his disciples through parables. Jewish teachers used parables as sermon illustrations to explain a point they were teaching (for examples, see Johnston 1977:507). To offer an illustration without stating the point, however, was like presenting a riddle instead (compare Test. Ab. 12-13A). By articulating his principles only in parables, Jesus offers riddles whose answer can be fathomed only by those who understand them in the context of his own ministry (for example, events like the Pharisees' rejection-12:24-45) or who patiently press into his inner circle to wait for the interpretation (13:12; compare Irenaeus Adversus haereses 2.27.3).
Jesus spoke in parables because the kingdom involved end-time "mysteries" (NIV secrets, v. 11) now being revealed to those with ears to hear. The disciples were more special than the prophets of old only because they lived in a time when they could receive a greater revelation than the prophets, as Jesus' blessing on them makes clear. The disciples' eyes and ears were blessed (v. 16) because of the greater one among them (v. 17). The rest of the hearers, unable to fathom his message, fulfilled the prophecy of Isaiah about penal blindness: because of Israel's sin, they would be unable to truly see, hear and understand God's message (vv. 13-15; 15:14; Is 6:9-10; compare Is 29:9-10; Evans 1981). Yet those who did turn to the truth would be "healed" (Mt 13:15); Jesus' physical healings were concrete signs of the spiritual healing of which Isaiah spoke (Mt 8:17; compare Is 6:10; 53:5; Hos 11:3; 14:4).
The disciples alone had pressed close enough to Jesus to understand the rest of what he was giving them. To those who had some revelation, more revelation would be given (Mt 13:11-12). In other words, the disciples alone proved to be good soil (v. 23).