The reasoning is based in PRACTICALITY. Binary blob drivers are a goddamn nuisance to work against since they are black boxes, you have no f***ing idea of what they are actually doing which is cause for system instability and also security. Not only can the kernel devs themselves update/modify/debug and quickly diagnose errors with in-tree drivers, it again also means that they can offer a vast driver support across ALL the architectures they support (there's more than x86) without having to rely on the goodwill of hardware vendors to supply drivers on non-mainstream systems.
Originally Posted by RealNC
And obviously it's paying off as more and more companies want to have their hardware supported by Linux and not having to maintain the driver themselves to prevent bit-rot while also having their hardware working out-of-the-box anywhere Linux is deployed is also a boon.