It can be due to a lot of reasons. One of my friends believes that he got rejected for a master's program (not SLP) because of the reference letters. He kept a copy of them because he ended up not applying to one of the schools. Sadly one of the referees whom he took a class from and got an A+ literally expressed her confusion as to why my friend would look for a reference letter from her. She didn't say any good stuff about my friend. This can be part of the reason.
It can also be about what every school values the most. For example, UofT and UBC value research experiences more, UofA values EDI experiences and overall profile more, etc.
From what you said, I think your profile is pretty competitive, so I have no idea what has gone wrong.
BUT, from someone in another thread, being rejected doesn't mean you're not good enough or SLP isn't for you. Sometimes you're just unlucky, with a lot of highly competitive applicants applying in the same cycle.