This is $25 or more over a cheap B450. I think for systems with a CPU like a 3100 or 3300x, PCIe 4.0 is not needed and the buyers would prefer to have a cheaper motherboard. I think MSI -max motherboards (mostly the cheap ones) will still fit the bill for these systems.
Also, the lack of support for 3000 series APUs on this is plain confusing, though I understand why the choice was made. If the box says "Ryzen 3000 Compatible" I could see how someone with a 3200g will be very unhappy when it doesn't work since the 3200g is a "Ryzen 3000" processor by name.
Honestly, AMD's motherboard situation is very confusing and will continue to get more and more confusing in many years on the used markets when you have no clue what generation bios is flashed onto a board.