alt Hacker NewsThere's plenty of use cases for small things which don't need any sorts of speeds, where you might as well have used a 115200 baud serial connection but ethernet is more useful. Designing electronics for 10Mbit/s is infinitely easier and cheaper than designing electronics for 100Mbit/s, so if you don't need 100Mbit/s, why would you spend the extra effort and expense?
There is also power consumption and reliability. I have part of my home network on 100Mbps. It eats about 60% less energy compared to Gb Ethernet. Less prone to interference from PoE.