Here is an
online PC Miler with HAZMAT routing, but it will likely give you the same routing that FECC gives you. But, more important, and maybe more helpful, is the
National HAZMAT Route Registry from the FMCSA. There used to be a routing program, but it was messed up and didn't actually pay any attention to HAZMAT restricted roads (your tax dollars at work). But, they have a current list of the HM Registry available for download. Check the Registry with the routing you have and see if there's a conflict with the routing you want to use.
I have PC Miler with HAZMAT Routing, and when I run it from Indy to Sacramento it does the same thing, I-70 straight out. 2402 miles, 39:36 hours.
But, if I force it to go up to I-80 (by inserting a stopoff at the Iowa 80 truck stop in Walcott) it will do that, but it'll also take I-76 right down to Denver and then I-70 right on over. 2463 miles, 40:31 hours.
But, if I add another stop at the TA in Rawlins, WY to make it go through both Wolcott and Rawlins, it will keep you on I-80 all the way to Sacramento. 2190 miles, 35:57 hours.
I have no idea. It's shorter and faster. Routing software does that some times. For whatever reason it doesn't always pick the best route, and you sometimes have to give it a little nudge to make it do what you want it to do. I'd just make sure to check the HAZMAT Registry to any newly added restrictions along the way.