The more I listen to this piece (to review for music history), the more I like it. There is a good amount of contrast - it begins with a forte descending arppegiation, then switches moods immediately to a smooth, mainly string section. This alternation between moods defines the entire piece, as there is rarely a crescendo or diminuendo - just sudden changes.
Another reason is its harmonic simplicity. Since the romantic era, composers have rarely used the harmonic progressions without contorting them in some way. Haydn stuck to the basics, though. For example, the first smooth string section is a parallel, symmetric period that uses the progression:
I ii6 V vi I6 ii6 V I
It doesn't get much simpler than that, nor does it need to to produce a satisfying aural affect.
Later in the piece, Haydn uses more involved progressions, but they rarely become unrecognizable. This keeps the listener interested, but still satisfied and not exhausted.