Firstly, because the repertoire progresses in a manner that reinforces small-step success in tone production, fingering and technique, musical points learned and musicianship. The songs do not ask the impossible of a beginning youngster. The songs are played with good attack, good air, good tone on shortish notes. Fingers move not too much at first. The progression then makes increasingly difficult demands on all of the production techniques a little bit at a time - but always by playing a song.
Students are always successful although rates of success are widely variant, of course.
It is as different as individual progress rates. Because flute starters are often older than the baby violin starters, I have found that 7 and 8 year-olds are often already reading bits of music in other situations. Sight reading from other books and when to learn the duet parts to the songs also are different from student to student. I have been doing this only for three years, and therefore do not have a long history of students who have gone on to other things. However, I know my Suzuki method students have excellent ears for music, play flute in tune with fine tone, display confidence in their playing, and have fun at lessons.