Because vi is installed on almost every unix or linux system you will encounter. Many of the other editors require explicit installation, which may, or may not

Because vi is installed on almost every unix or linux system you will encounter. Many of the other editors require explicit installation, which may, or may not be possible depending on your bureaucracy and where the system is running in your infrastructure. If you know vi/vim you can work on files everywhere. Using the same editor everywhere is much less confusing than trying to learn several different editors and mentally flip back and forth between them.