Simple supply and demand.
To build up a tubular K frame, you need to build a jig to hold all the tubing in place while you weld it. You also need to design all the geometry around it. All of this work is the same regardless of the make of car. Once it is all said and done, you now need to weld it up and fill it out with parts. Up to this point, everything is equal. Once that is complete, you amortize the cost of the start up against the projected sales volume, available competition, and add in your mark up. Unfortunatly for mopar guys, this is where things all fall apart. Ford and Chevy guys have a ten to one advantage over us in sheer volume. Now, you take that paltry one and start disecting it and it is 45% resto crowd, 45% drag race crowd, 5% handling crowd and 5% rat machine/daily driver/misc.
Additionally, when you are manufacturing a run of products, it is much easier to perform all the set up and run all the parts through in volumes. Odds are pretty good if you build a chevy tube frame, you can build 100 of those at one time and sell them all over the course of the year, so your total build cost goes down with the increased volume, increased buying power for base components, and decreased set up time. By contrast, if you build 100 mopar units to keep the cost down, you will sit on those units for the next 10 years before they are all sold. This means you now have carrying costs, inventory taxes, and potential storage fees or lost opportunity costs because they are taking up space in your shop. Also add in to that
Does it suck, yeah. Its even worse for AMC or Studebaker guys, but no where near as cheap as chevy or fox mustang segments. Ultimately, being unique has its costs.
What to be overwhelmed by choice, go to Summit and look up chevy header bolts.