Mary,
I think the answer to your question has several elements not all related to CK compared to all other possible treatment options:
1) The true medical advance of CK is really not related to AN's but the ability to provide a true radiosurgical option to tumors beyond the skull. If one considers the huge number of people having CK for lung, prostate, Liver, spine, stc tumors as protocols are developed it's utilization is pretty dramatic beyond the AN community we all see here.
2) Of the people who have the choice and take the time to explore both AN treatment options, each makes a personal choice based on the information they are given and what sources they get them from, some choose surgery , others radiosurgery
3) Of the people who choose radiosurgery , not all take the time to research the differences between the various machines or are limited to the machine available to them based on the doctor / medical facility they have selected or insurance. GK , Novalis and others are also effective but give up certain features. GK is very accurate but lacks the ability to do FSR and requires a frame. Others can do FSR but sacrifice accuracy compared to GK. CK combines the best of both essentially.
4) I have been participating in this board for almost 5 years and for a while I was the only CK patient and one of only a few radiosurgery patients on it. There are certainly more of both that have actively participated in the past couple of years. Shoegirl, Phyl, and Jamie come to mind quickly as other CK patients that post. I would guess there are around 15-20 folks I know of that started on this board when they were diagnosed and were treated by CK but are no longer active. Fundamentally, this forum serves two communities of AN patients. Those that are newly diagnosed and looking for help in choosing their option and those who have been treated and are looking for support or suggestions on post treatment issues. Based on my observations, very few of those treated by radiosurgery of any type seem to fall in that latter category. So, my theory would be that those folks tend to move on with their life and lose interest in staying involved in a forum like this.
It's hard to prove any of the above scientifically, but it reflects my intuitive belief based on reading posts here for a while.
Mark