Ready for a long winded story to better understand how we got to where we are?
In the early days the rules read 2 by 3 of .120 1000 series steel or it's equivalent. This meant you could switch to round tubing or lighter gauge rectangle tubing if it was larger, or if you changed alloys that had more strength, they could be smaller thinner and lighter. I personally seen a stainless frame Gasser under these rules. The T/F and AA/F jumped right on this because they ran weight to cubic inches then. A 400 inch engine was big, so the ability to get rigidity and light weight was more than anyone could pass up. By the 70s if you didn't have a C/M chassis on a dragster, you didn't have a good dragster. From there every one figured if the big boys have it it must be good. What they over looked in the equivalence thing was while it did have stronger quality's it had short comings also. The difficulty to weld properly may be the biggest but being brittle it was prone to work hardening and cracking with just normal use. It also transmits force threw the frame very well. This is hard on the driver and if there is a spot by the weld that has changed temper you have a good place to fail. That rigid steel is transmitting force right to that weak spot.
I was always under the understanding that 4130 changed hardness at 660 degrees and it takes 1200 to get a puddle going. Now your welder manufacture says TIG weld it but heat it slow up to the puddle then weld it slow and cool it slow. the bead should be convex not concave (that's an outie not an innie) and a gusset on joints is advisable. The FAA says oxy/ Actt. torch is the preferred method. This makes your weld go slow. Don't try this at home, they pay more for their torch body and tip than we would for a good HVLP spray gun. Some swear by preheating and wrapping with asbestos wrap for slowing cooling. Depending on how hot you got it and how slow you cooled it, it could anneal it. This is softer. They sell an annealed 4130 it is slow cooled in a furnace.
I have a friend that was a Maxim sprint car chassis dealer and he repaired them. I sometimes helped him when he was behind. the failures was always was with in an inch of the weld. These are top of the line chassis. that has to tell you something. He also restores old open wheel cars. they where made of mild steel. They bent and crumpled up, but did not fail.
Some of this could be how it was put together. Where the intersections where if there was a gusset ect.
Dragsters with long runs of tubing and using the chassis for a suspension may need 4130. But does a door car?
The fact is there are more alloys available than 1000 and 4130. If all buildings aren't built the same, why should race cars. That's engineering.
I sometimes believe the rules committee has self serving motives as they are all chassis builders. There is an article that says a couple guys was dismissed, as they had no dog in the fight. Meaning they where not active chassis builders any longer.