Why 2. c4, and not 2. Nf3, for beginners?

I think that's at the heart of Schiller's recommendation of the Bishop's Opening.  It's the choice against 1...e5 in three White repertoire books(that I know of) from three different decades:  Soltis' Winning with 1 e4(1988),  Schiller's White to Play 1 e4 and Win(1992), and Emms' Attacking with 1 e4(2001).  The Soltis book pushes a system with an eye toward transposing into the King's Gambit Declined,  Schiller's book looks to transpose into the Scotch Gambit, and Emms' book advocates lines with an early Nf3 but also uses the KGD if need be.  Paring down the opening is a high priority in one volume repertoire books like those.  I don't know the Schiller book that you have, but it sounds like he's of a similar mindset in that one.