A) the test had outcomes that heavily favored teachers who grew up in Ontario and spoke English. Part of this is that there were no prep materials available at the time and no real description of what was on the test
B) the province failed to demonstrate a need to use this particular test or that tests like this led to better student outcomes.
For reference on A, here are some sample problems:
Question 1: "Based on Learning For All, which is a shared goal or strategy of differentiated instruction and Universal Design for Learning? A) To use the same textbooks B) To use only whole-class instruction C) To meet different needs of the students in a class D) To meet the requirements for curriculum expectations"
Question 6: "Assessment that occurs before a period of learning to assess a student's readiness for learning and learning preferences would be considered which of the following? A) assessment of learning B) assessment as learning C) assessment for learning D) assessment of learning skills"
Question 15: "As outlined in Growing Success, how does Ontario’s criterion-referenced assessment and evaluation approach require teachers to assess and evaluate students’ work? A) by ranking student performance B) by comparing work done by other students C) by referencing performance standards developed by teachers for their own classrooms D) by referencing the established criteria for four levels of achievement that are standard across the province"
Those aren’t basic math knowledge, those are trivia on the Ontario curriculum (and again, no prep material was available). If the province had demonstrated that this improved student outcomes the ruling may have been different.