Why driver deinstallation or update require system restart?

Filesystem filter driver: In most cases, if the driver is in use, it cannot be fully stopped without a system restart. The reason is that the filesystem filter driver allocates private blocks of information that are attached to opened files. So, if there exist open files, the driver must wait till they are closed and dispose of these allocated private blocks. If the driver is unloaded, then either the memory leak or even a crash can happen.

CBRegistry and CBProcess drivers work differently so they don't need to wait for anything and can be unloaded.