Because

John picking out a part on the Moog synthesizer

The very last song that John Lennon offered to the Beatles was this beautifully simple composition.  The story goes that he had been inspired to write it after hearing Yoko sit down at the piano one day to play Beethoven's famous Moonlight Sonata.  Perhaps he already had an arrangement in mind while writing it, because the recording was uniquely suited to the Beatles' capabilities.

Although Ringo does not appear on that recording, he played an integral part in the process, tapping out a beat on a hi-hat that John and producer George Martin could hear in their headphones as they played the basic arpeggios on guitar and electric harpsichord over and over again.  Paul also played a bass line on those twenty-three takes before the tape was rewound and they decided that take sixteen was the best.  The group then moved on to the most intricate part of the track - the three-part harmonies sung by John, Paul and George.  All of the above work was completed on August 1st, 1969.

The group returned to the track on August 4th, overdubbing the three-part harmonies two more times to end up with a total of nine voices on the master.  Getting the vocals just right took a great deal of time, with George Martin guiding the boys as he had in their early days on Lennon's other three-part harmony compositions This Boy and Yes It Is.  The precision they achieved here was so perfect that years later, in 1996, this song would appear on Anthology 3 in a vocals-only version for listeners' maximum appreciation.

The final touch was added on August 5th.  George Harrison had brought his Moog synthesizer to Abbey Road Studios and had it set up in a small room for the group's use.  Wires were run to the control room and George played a few brief parts on the synthesizer's keyboard as overdubs for the middle and end of the song.  This new instrument was introduced with such subtlety that it did not sound out of place among the guitars and electric harpsichord.

Because was not considered to be part of the long medley on side two of Abbey Road, but its final chord hangs in the air in such a way that it perfectly sets up the opening of Paul's You Never Give Me Your Money - yet another brilliant decision by Martin and the group when it came time to select the sequence of songs for an album.