Why Disable all interrupts was allowed in kernel mode?

It usually doesn't. You may think of one of the followings:

1. When processing a higher priority interrupt, more interrupts with lower priority are not allowed.

2. During the context-switch (user-mode<->kernel-mode, entering/exiting interrupt/exception handler), interrupts may be disabled.

3. When in a critical operation (eg. manipulating shared data), interrupts may have to be disabled (for a very short time-interval).