The big benefit of Al is that it can be formed more easily than steel, to make a lighter bike with the same strength, which for lower-end bikes is a benefit.

Steel, to match AL's weight, needs to be weaker or needs to be very carefully designed and constructed. Al can be shaped a lot more easily and machine-welded (so can steel, but it is hard to weld very thin steel.)

Al can be formed to have better ride characteristics, meeting or beating steel at a lighter weight.

So ... if a factory wants to make a cheaper, lighter bike, Al is the best choice. And since people are seeking lighter bikes (which wasn't a big deal 30-40 years ago when steel was king and Al was in its infancy)