It is easy to catch an indentation error with two spaces, while it is harder to make an indentation error with just two spaces. It also easier to type two space

It is easy to catch an indentation error with two spaces, while it is harder to make an indentation error with just two spaces. It also easier to type two spaces than 3 or 4 spaces, which is important when program is typed without help of an autoindentation tool. Two spaces are enough for fixed length font, typical in terminal, but also saves screen space, which is just 80 characters in width.