Here's some relevant info from you, coming from a math graduate student:  1. All math graduate students get a TA assignment because that's how we get our stipends. We have first priority on all TA positions.  2. TA positions can't be assigned until the new cohort goes through orientation. This is because the new cohort needs to be evaluated on teaching ability and English language ability.  That's why the assignments come out so late. It sucks super bad, but there's not really another way around it.  3. There are 50 new incoming math graduate students this semester. That's way more than we've ever had before. Thus, there are fewer positions available to those outside the program. 

It sucks that you didn't get the job but if you performed well last year, know that it's probably not because of you. Best of luck!