The reasoning behind most things is selfish. Even Richard Stallman is subject to this; his motivation for the free software movement was when a colleague wouldn't share software changes that Stallman wanted/needed for his own use.

That said, the specific objection of Linux to the GPLv3 is twofold:

* There are thousands (if not millions) of contributors that would need to be contacted and would need to give consent to such a licensing change. I imagine some of those contributors might even be dead, and given the currently-draconian copyright laws of many nations (particularly the United States), this means that relicensing is that much more difficult.

* The GPLv3 would be a very hard hit for a lot of users of Linux in the embedded space, where a large number of "Things" in the "Internet of Things" aren't exactly designed to be field-programmable. The anti-TiVoization wording in the GPLv3 would cause a lot of pain there.