Chromoly has always been the most popular material for BMX frames. Even at the time when road and mountain bikes rapidly converted to aluminum, BMX followers remained resolute in their preference for steel.
Impact resistance is one of the most coveted characteristics of a steel BMX frame. The more adventurous your street riding becomes, risking larger drops and curb impacts, the greater a likelihood of crash damage.
Steel is more resilient to crash damage and deformation than aluminum, which does not have the same ability to absorb riding errors of judgment and return to form. If you do have a significant impact crash with a steel BMX frame, a skilled welder can easily repair it – whereas an aluminum frame is much more challenging, and costly, to fix.