Most Linux distributions ship with a certain kernel version and only update it on point releases (
x.y.(z+1)) and for security updates.
On the other hand, I know that Linux has a very strict policy about not changing the kernel ABI and never breaking user space. In fact, Linus have had many public tantrums directed at developers who wanted to (intentionally or accidentally) change the kernel is non-backwards-compatible ways.
I don't understand why distributions use "stable" kernels instead of always updating to the latest kernel. This is not a criticism, I'm just curious about the reason.