Bo-
4130HT Is has a very good looking tensile strength when that's all you pay attention to. However it is very brittle and low impact strength and fatigues relatively fast.
That material was required as opposed to 4130 Normalized (4130N) which has a lower tensile, but the other mechanical properties are much better for the severe service of a race car chassis. The aviation industry outlawed the use of 4130ht back in the 30's, so it wouldn't have taken much research on the part of the builders and/or sanctioning body to know better than use that grade of material. By the time they figured out that there was a big problem nearly every car running was built with 4130ht, "what a deal!"
The biggest problem as for as I'm concerned is that once the issue was figured out, no one had any idea where or why the requirement came from in the first place.
That's why I tend to question folks when they start demanding things that don't make good engineering sense, in the name of safety. Is it really safer???
Rouse