Well, if the intestine gets inflammed enough from diverticulosis, that causes the bowels to swell up big-time, and that swelling has to be taken care of before the doc can go forward with a colonoscopy. This is why they are giving you a barium xray and CT. They want to SEE via film, rather than with an object pushing through, what is going on in your bowels. Then they can give you medicines or more involved treatment to clear up or at least control any intestinal inflammation you got going on. The reason the doc went straight to naming your digestive issues diverticulosis, could be as he went through the beginning of the large intestine, he may have seen some corrosion there, and when he couldn't make it further along because of inflammation and swelling, then it was an automatic leap he took in concluding diverticulosis. I might add that the docs are going to want to know how extensive this disease process is, and also, they'll be able to "finish" the colonoscopy, as it were, thru the imaging techniques they have set up for you.
The colon from beginning to end is composed of ascending, transverse, descending, and sigmoid colon, followed by rectum and anus. The gastroenterologist starts from the anus and works backwards. Oftentimes, the sigmoid colon (or first bend as your doc called it) is tortuous, resulting in an incomplete colonoscopy.
Alternative imaging modalities: Barium enema is old school and less frequently used nowadays. A better option would be CT colonoscopy (virtual colonoscopy). You still have to do the bowel prep, unfortunately. Aside from having to insert a tube into your rectum to inflate your bowel with gas, this test is otherwise noninvasive and does not require sedation. You are basically getting a CT with your bowel distended and clear so the radiologist can look for suspicious polyps. The one downside is that if there is a polyp you will still need a colonoscopy to biopsy it.