Because they're fundamentally different things? A house is a machine for providing weather protection. The difference between a modern house and an old one is pretty minor. A computer is a machine for doing calculations. The difference between a modern computer and an old one is - by more or less any metric you can think of - many orders of magnitude. Calculations per Watt, calculations per second, calculations per unit volume etc.

It's not even like we're breaking them. This is just the maintainers of the Linux kernel choosing to not spend their time maintaining compatibility with the old architectures.