With 2 backs you either got 1 less WR or 1 less TE. You got to have 4 behind the line of scrimmage, so 1 receiver or TE needs to take a step back. Sometimes one of the backs will go in motion and line up as a WR or TE.
With 2 backs you also got 1 less back than a 3 back set, where everybody else would have to be on the line. 3 backs takes a big front 7 to stop with everyone being disciplined and being aggressive toward their assignment.