It's probably the most researched, well known, and the most used material in just about every industry. It's the most tunable, in the sense that if you want to tune a frame the way someone tunes a piano, you have a really great selection of tubes that you can select from.
Steel is also generally the most affordable of the materials listed here.
Paolo talks about working with steel. Even with the thick wall, the aluminum tube is lighter. You can't really let it flex, because the more it gets to bend the quicker it reaches the end of its life. That's why you see a lot of aluminum frames today that have very large diameter tubing. That's to limit the flexing that happens as you ride the bike.