Diatonic vs chromatic harmonica

Question

The hohner super chromonica has the pattern

Do re    mi fa    sol la    ti do
^     |       ^    |      ^   |      |   ^
 |    v       |    v      |   v     v   |
____         ____       ____     ___
  1             2            3         4    <——Holes
  5            6            7          8
  9           10          11        12

The hohner marine band harmonica has this pattern on holes 4,5,6,7 but due to less holes has different patterns on holes 1,2,3,4 and on 7,8,9,10

In the lower range there is a duplicate note on holes 3,4. (3 draw is sol.) Why wasn’t it made fa?

Answer 1

The low 4 blow notes give you the I chord: 1, 3, 5, 1.

The low 4 draw notes give you the V chord: 5, 1, 3, 5.

Putting in the 4 would give you a rootless dominant 7.

Answer 2

At a guess, the prominence and importance of the 5th in pentatonic scales might have something to do with it; as I'm sure you're aware, diatonic harps are also called 'blues harps', and blues music generally focuses heavily on the dominant (and slightly less on the 4th). From Wikipedia:

Although there are three octaves between 1 and 10 "blow", there is only one full major scale available on the harmonica, between holes 4 and 7. The lower holes are designed around the tonic (C major) and dominant (G major) chords, allowing a player to play these chords underneath a melody by blocking or unblocking the lower holes with the tongue. The most important notes (the tonic triad C–E–G) are given the blow, and the secondary notes (B–D–F–A), the draw.