Support for MediaTek's chips would help Windows Phone quickly gain a lot of users but it seems Microsoft decided to keep it Qualcomm-exclusive for now.
http://www.notebookcheck.net/Windows-Phone-8-1-to-support-only-Qualcomm-processors.114860.0.html
I hate to give Microsoft credit on something where they limit the freedom of the windows platform... But based on my experience with my Android devices, this is the right move to make. Windows phones can be a middle-ground between the open-ness of Android and the streamlined/reliable experience of Apple's iOS devices. By supporting only one type of hardware platform, it becomes infinitely easier to optimize and avoid bugs/crashes especially with 3rd party apps.
Interesting opinion, but do you really think this is about avoiding bugs/crashes, thus decreasing the overall reliability of the platform? I only think it's a matter of time until Microsoft makes the system available to all platforms. After all, if Intel allows cheap tablets and convertibles from China to use their processors...