12.19Why C7 is unusual in C major

C major tonic chord is:

C E G

But C7:

C E G B♭

contains B♭.

That B♭ points strongly toward A:

B♭ → A

which is part of F major.

So:

C7 → F

means:

V7/IV → IV

The tonic itself temporarily becomes a dominant chord.

This is a very common sound.