First, Linux does not require contributors to assign copyright to some central person. Copyright on it is spread out among many, many people (including some who are dead, in which case much effort would be required to figure out who inherited the copyright). Any of them could stop a license switch, unless someone goes through and removes all parts of their work from the kernel. Because GPL v2 and v3 are incompatible, it is illegal to release Linux under v3 with any contributions licensed under v2 only.
Second, Torvalds personally does not like GPL v3. He particularly dislikes certain provisions (like anti-tivoization), which are not restrictions he wants to impose on users of his software. As he won't release his stuff under v3, the whole kernel can't be released under v3 by anyone without prohibitive effort. But he is not the only person who could singlehandedly make it impractical to release the kernel under v3.