4xx chipsets added a few things, but one of them is the improved boost clocks. 3xx chipsets wouldn't boost and idle as good as 4xx, so there was a good reason to go for the newer one.
5xx chipsets added PCIex 4.0, which is a big jump over 4xx. Also, PCIex 4.0 needs a motherboard that can handle it, to allow that speed on older 3.0 boards would cause trouble for the user, who in turn would see their computer crashing randomly or other problems.
B550 has improved connectivity over B450, and also PCIex 4.0. But B450 is still on the market, and is compatible with every Ryzen released, so you can just save some money and go for the older one.
Intel has very fast CPUs, but in terms of motherboard support, AMD is way, way better than them. You can't put an i9-9900k on an H110 motherboard, even though both use LGA 1151, can you? Because that's what AMD is doing.