Breyer has been misquoted more often than quoted, but 1. e4 is more or less of a pawn sac seeking to exploit the unfortunate position of Black’s king in the opening array. If Black replies 1. e5 the drawing lines are legion.
Alternatively, 1. e4 can reasonably be answered by 1. … c5.
So White plays 1. c4 and Black answers 1. … e5.
1. d4 dodges these tradeoffs by advancing a protected pawn to a center square on the gravitational, if not geometric, center file of the board in the opening position.
When the center resolves in the e4 games, there typically follows a head-on clash in which White, perhaps a pawn down, manages a draw by dint of superior activity born of White’s space-time advantage in the e4 games. The activity is in the form of constant combinatorial threats leading to perpetual check or conversion to an unwinnable ending.
When the center resolves in the symmetric d4 games, White’s first move has typically yielded an intrusive presence. By means of mild combinational threats (e.g., a pawn win) the intrusive presence is maintained through conversion after conversion while Black struggles for neutralizing exchanges.
In the d4-e5 and d4-c5 openings where Black challenges White’s blockade via direct contact, the resolution in the center leads to an asymmetric struggle that looks at first glance perilously close to won for White. But it’s not really true: The 20th century main line of the Orthodox KI with Bd2 is so resoundingly drawn it has become abandoned in grandmaster practice. Whereas the popular variation in current Orthodox KI practice, Be3, appears objectively to present to White a slightly less attractive position calculus than the Bd2 line.