Why doesn't Snape get DADA?

[Edis]

No: HPFGUIDX 123755


There is a plot-structural reason why Snape might get DaDa in HBP.

Harry still (we surmise) has the ambition to be an Auror.

To do this he needs Potions at NEWT level. 

We have Canon that he felt he did better at OWL potions than he 
expected but it is unlikely he will get a top grade and we know Snape 
is implacable about only accepting top grades into his Potions NEWT 
classes. I can't see Jo pulling an academic miracle and giving Harry 
that grade...

But if Snape becomes DaDa teacher this year then there is scope for a 
new potions teacher to be more lenient and accept Harry into class.

And maybe Snape will show some actual Dark Arts in his new class... 
at least for NEWTs. Given the war situation the extra aggro Snape 
would cause in class could be justified by the need to get people up 
to speed in advanced techniques.  

One consequence, the new teacher would be a potions teacher.