#2 Sane's performances for the national team
Okay, this might be a little contentious but Sane doesn't reciprocate the form he has for Manchester City when playing for Germany. If Low based his selection merely on performances for his team, there is no reason why this decision is contentious.
It has a lot to do with tactics too - Sane is given the freedom at the club level to move forward freely and take shots, but Low's structure doesn't work like that. Sane has very rarely delivered when playing for Germany even during his time before City.
In the most recent game against Austria (that Germany lost 1-2), Sane was horrible and that might have been the last straw that broke the camel's back.
But this is not the end of the line for Sane. He is only 22 and will get much more opportunities in the past, but not before he starts raising his performance for the National team.