There is no good reason - it is purely customary. You may use any symbol to represent a variable - including characters from other alphabets. For example, in geometry and trigonometry, Greek letters are often used to represent variable angles.
The word alphabet is formed from alpha-beta, the first two letters of the Greek alphabet. So saying alphabet is pretty much the same as saying ABC.
after ABC is DEFGHIJKLMNOPQRSTUVWXYZ
English: "the alphabet" is German: "das Alphabet".
Because a,b, and c are the first three letters of the alphabet
Alphabet.
abc... is the alphabet.
Yes.
ABC
This is the English language alphabet. ABCDEFGHIJKLMNOPQRSTUVWXYZ.
The Abc
get a dictionary with an abc alphabet of braille codes
ABC