The premise isn't really true, since such hexatonic scales are actually very common in folk and country music. In particular, the “missing **ⅶ** scale” 221223. An important source are Scottish tunes, e.g.

    X:1
    T:The Athole Highlanders
    L:1/16
    M:6/8
    K:D
    V:2 clef=treble
    |: A6 A3FD2 | A3FD2 E3FG2 | A6 A3FD2 | E3FG2 F3ED2 |
    A6 A3FD2 | A3FD2 E3FG2 | Ad3A2 B3AG2 | F3GE2 D6 :|
    %

Celtic music is strongly dominated by a single melody, which is often _mostly pentatonic_ but adds in the remaining diatonic notes occasionally. 

This tendency to keep a lot to the major pentatonic (as well as the Blues one, though that is actually quite different) has had a lot of influence on American Folk and through that on country, rock and pop music. However, those genres have much more emphasis on a chordal accompaniment, using the **Ⅰ** and **Ⅳ** chords most often, so country can hardly be called pentatonic. Hexatonic it is.

The situation is very different in classical music. This too is dominated by melodies, but there is a fundamentally different approach. Whereas Celtic music likes a “constant floating feeling”, classical music is all about building up, constructing cadences that have clear resolutions. And the most powerful melodic resolution is the **ⅶ**-**Ⅰ** step, which is therefore all over classical music.

But even classical music may use hexatonic scales, in particular when going for a folky feel. But it's usually limited to short motifs:

    X:1
    T:Sinfonie No. 6, “Pastorale”
    C:Ludwig van Beethoven
    Q:"1: All. ma non troppo"
    L:1/8
    M:2/4
    K:F
    V:2 clef=treble
    z .A.B.d | (cB/2A/2 .G).C | .F.G (AB/2A/2) | .G.A (Bc/2B/2) | Accc | c2
    %

A bit longer:

    X:1
    T:Finlandia
    C:Jean Sibelius
    L:1/4
    M:C
    K:Ab
    V:2 clef=treble
    z (c B c) | (d3 c) | (B c A z/2 B/2) | B (c3 | c) (c B c) | (d3 c) |
    (B c A z/2 B/2) | (c4 | c) ( !tenuto! e !tenuto! e !tenuto! e) |
    (f3 c) | (c e) (e3/2 B) | B (d3 | d) ( !tenuto! d !tenuto! c !tenuto! B) |
    (c3 A) | A (B2 z/2 A/2) | A
    %

I'm a little hesitant about that example, because the theme doesn't span an entire octave, and it's homophonically harmonised with voices that do include the **G** note. So it might well be argued that it's actually heptatonic but simply keeps in a small range of the scale.

A hexatonic mode that's rather more natural for classical music is the “missing **Ⅳ**” 223221 scale. A prime example of this is the gorgeous Andante Maestoso from Holst's Planets:

    X:1
    T:Jupiter, the Bringer of Jollity
    C:Gustav Holst
    L:1/8
    M:3/4
    K:Eb
    V:2 clef=treble
    z4 G,B, | C2 (CE) (D3/2B,/2) | EF E2 D2 | CD C2 B,2 | G,4 G,B, |
    C (CE) (D3/2B,/2) | EF G2 G2 | (GF E2) F2 | E4 (BG) |
    F2 F2 EG | (F2 B,2) BG | F2 F2 GB | c4 cd | e2 d2 c2 | B2 e2 G2 |
    FE F2 G2 | B4
    %