To start with my conclusion, there are plenty of flat keys in Suzuki books before volume 5, so I wondered what the teaching moment of the Concerto in G Minor could be. I watched the CadenzaStringsNC guy play the piece. My guess is that this piece is "held back" until book 5 because it looks like it involves shifting competence into a few higher positions. 

I looked back at my Suzuki book 1 and realized that it only introduced a few # keys.
Book 2 gingerly introduces F and Bb Major.

For the record, here you can find the contents of the Suzuki books 3, 4 and 5 to see which pieces (and in which keys) are included:
     Book 3: http://www.amazon.com/Suzuki-V.....0739048155
     (includes: Gavotte in G Minor (Bach))
     Book 4: http://www.amazon.com/Suzuki-V.....0874871506
     (includes pieces in A and D Minor)
     Book 5: http://www.amazon.com/Suzuki-V.....0874871522
     (Concerto in G Minor, Op. XII, No. 1, F.I.N. 211 (Allegro, Adagio, Allegro)(Vivaldi)

Here's the Cadenza guy shifting his heart out in the 2nd movement (a link to the faster 1st movement is below the video):
https://www.youtube.com/watch?.....5xolssAppQ

That's my guess.