QTM & Halting problem
Because we can simulate QTM with DTM and it is an easier proof method than proving directly for QTM (which we can do). Also, this method of simulating a model with DTM to prove they are equivalent in power is somewhat a standard tool, not just for various TMs, but also for other computaitonal models ($\lambda$-calculus, RAM-machines,$\ldots$)

I think this question and answers address everything else you may be wondering


References on comparison between quantum computers and Turing machines