Because they are completely different procedures.
A colonoscopy is the examination of the colon using a CCD camera or fiber optic camera on a flexible tube inserted through the anus.
They attach the end of your intestine to a hole in your stomach instead of anus if too short to reach after bad part cut out, then you have to wear the bag for waste collection. It's a serious operation, what about it specifically are you looking to find out your md hasn't told you and hopefully it is not needed. I am surprised they are assuming without going in for exploratory first, hopefully they are telling you it is only a possibility. In any case, get a second opinion no matter how good your md is if this might be before you bite the bullet