why only greek letters are used as a symbol , like Alfa bita, Sigma & Gama?

Answer 1

First of all, much of our mathematics comes from the Greeks. Also, the letters look very different from ours. If we used Russian, for example, their V looks like our B.

Also, traditionally, learned people studied Greek and Latin, so they would be familiar with the letters. If you said "Alpha," for example, they would know what it looked like.

Answer 2

These were introduced a hundred to two hundred years ago. This was what was in fashion at that time to use Greek letters.