It was probably locked because last year's sheet contained a lot of incorrect information and ... shall we say ... unflattering commentary about certain programs, program coordinators, and program directors. The coordinators got wind of that and spread the word. Even tho all comments were anonymous, the programs were also able to discover the "game" that some students (certainly not all) were playing with holding onto interview spots they were offered until something better came along - often cancelling those initial interviews with so little notice that the programs were not able to easily re-fill those interview spots. Programs were not very happy about this; it's possible programs will not be as forgiving with those last-minute cancellations anymore. Those students (again, not all) don't seem to realize the amount of work that programs put into having an organized, quality interview day, and how disruptive to a schedule that can be.
It's a shame that this year's spreadsheet was locked, really. I am a coordinator, and I have occasionally popped onto the old SDN discussion threads, and the spreadsheet last year, always with the transparency of announcing I'm a coordinator. I would answer some of the behind-the-scenes questions and try to calm the anxiety that I would see among the applicants. (For instance, there were comments last year along the lines that all the PDs get together at AAO or have other secret meetings to discuss the applicants and divvy up which program will invite which students. Nothing could be further from the truth - there's no way PDs have the time for that, yet it seemed to be a very serious idea!)
So my parting advice to you is to take the commentary on that spreadsheet with a grain of salt. Be genuine and honest in your dealings with the programs. And remember that the coordinators are people who are extremely busy in the fall, and may not be able to respond immediately, or they make a mistake now and then. We are preparing for your interviews, which is a big task, but we also have our normal daily job duties as well. Your PD and faculty teach you the medical side, your PC is usually the person to help you make it through the administrative side of training as smoothly as possible.
Good luck!