The naming of notes is part of the western tradition of music.
The notes in the scale are named A,B,C,D,E,F,G, then back to A. These correspond to the notes in the Do-Re-Mi scale: A is La, B is Ti, C is Do etc. These makes sense as the notes from A-G make a Minor scale, and the notes from C make a major scale, there is either one tone, or one semitone between each note, and on a piano keyboard these are the white keys.
A Guitar is not tuned A,B,C etc because this would make it harder to play chords. A keyboard has no way of changing the pitch of a note, so it necessary to have one note per key, one key per string. A guitar has a fretboard, so to play an F, you just fret one up on an E string. The choice of EADGBe is for convenience: most major and minor chords can be fretted quite easily.
It is possible to tune the guitar differently (people have written books about this). Each different tuning has advantages and disadvantages. Other instruments use different tunings: A violin uses G D A E. The pitches are more widely spread, and this is more useful for a smaller instrument.
A B C D E F G would be a particularly useless tuning, as the pitches are much too close.