I guess because mathematical formulas usually use single letters for symbols. It is so common that you end using several different alphabets, lower/upper case and even calligraphic variations. Of course it doesn't scale when you need thousands of symbols and your variables doesn't have well established meanings like "magnetic field" or "pressure". However they are used to it and it's hard to break some mental models after several years of using them everyday. For good or bad some scientific computer languages (like Julia) encourage you to use the Unicode alphabet to align your code with your paper/book.