Q:

Merge [linux-kernel] into [linux]

Because Linux is the kernel.
(A full operating system based on Linux is not normally called Linux; it's GNU/Linux)
From the tag wiki (tag usage section), it seems linux is meant to be used for questions about the Linux (not about things that may or may not come together with Linux), therefore these 2 tags are duplicates of each other.
Since Linux is, by definition, the name of the kernel, -kernel is unnecessary, so the merged tag should be just linux.

A:

Strictly speaking you're correct...but the tag wikis for both beg to differ.
For linux:

The linux tag on Stack Overflow is generally used for questions
  about:

programming against the APIs supplied by the operating system
the implementation of the kernel
programming tools and techniques for use on a GNU/Linux system.

Notable questions that probably don't belong are:

questions about using a desktop or laptop GNU/Linux system are better directed towards Super User, Unix & Linux, or Ask Ubuntu.
questions about administering GNU/Linux systems for other users and networking GNU/Linux systems in a context more complicated than a
  minimal home network are better directed towards Server Fault.

For linux-kernel:

This tag is for questions about the internals of the Linux kernel
  itself - particularly about writing code that runs within the context
  of the kernel (like kernel modules or drivers).
Questions about writing userspace code in Linux should generally be
  tagged linux instead. Since the internals of the Linux kernel
  are constantly changing, it is helpful to include the precise kernel
  version(s) that you are interested in.

...So despite the fact that there is a technicality in play here, the tag is incredibly well disambiguated.

