It's also possible that left-side entry for big planes became standardized because of the demands of airports with jetways. The first airport to install jetways perhaps decided to have pax enter on the left so that the gate areas would have jetway-plane-jetway-plane and so forth. Otherwise a left-jetway net to a right-jetway might not leave enough room in between for two planes to fit at the same time. Once adopted that could have become the standard -- certainly it's fixed by the design of the jetway itself.