Can you tell me what program you applying to?
My initial impression is the school is using the waitlist as "applicant purgatory", where no one is rejected initially and everyone who does not receive an acceptance is placed on said waitlist....sorry, that's probably not the answer you wanted to hear. IF the interview is not optional, part of the application process to be accepted and the program omitted you from that particular step; yes I would be concerned.
Your best bet to see where you stand with the program would be to call the admissions office and ask if the cohort has been filled, what the reply due date is, and if they rank the waitlist and what position you are?
Best of luck.