It’s actually 4.0, my bad.
I don’t think Nintendo did it on purpose. I think the older OS’s require the use of an IOS that is stubbed out on 4.0U and up. When Nintendo updates the System Menu, they only make changes to the individual IOS’s that need updating and when they’re done using one they stub it out so if you don’t go back far enough you’ll put a System Menu version on there that requires an IOS that is empty.
The last time I downgraded I went back to the pre-launch OS and updated one version at a time until I got to 3.2U. It took about 30 minutes on WCU’s internet but everything worked great. Tbh, I think if you went ALL the way back and went up one OS at a time you’d still be able to get the newer Wiis to run SM3.x. Rolling backwards or just installing the OS you want, as most guides do, is extremely dangerous.