Originally Posted by Trakhak

It's not surprising that the occasional weld failure is reported. Aluminum frames have represented the vast majority of adult bike sales since at least the mid-1990's,

Aluminum bikes replaced steel bikes in the market for a simple reason.

Steel bikes can be moderately durable and somewhat affordable (and heavy; Surly's hipster marketing made heavy steel frames an acceptable alternative to aluminum) or moderately light and expensive.

But using aluminum made it easy for manufacturers to provide affordable frames with a good margin of strength while keeping the weight reasonably low.