I wrote this reply while you were adding yours but I'll submit it anyway:
I would speculate that certain tools are necessary and need created or perfected for QM to be used properly (such as the item editor which is usable now but still a work in progress). A Quest creation game would be rather limited without things like the item editor.
Also, I'd guess once they do launch QM they will likely want as many devs as possible to handle the unexpected rush of issue certain to arise with something so new and unique. Thus, the desire to tie up current projects with TM & SM.
Of course as I said, this is just my speculation, but it seems logical.
I'm excited to see what QM will be like and assume and hope it is still in the future plans.