Why could my doc not easily remove a 1/2 inch sessile polyp on the sigmoid colon? what type of doc can do it?

Sessile = flat: Sessile polyps are flat, which makes it very difficult (but not impossible) to remove. However, they can usually be biopsied to determine if (pre)cancerous or not. This is where the gastroenterologist comes in handy. Realistically, it comes down to experience as newly minted docs may not have skills of seasoned ones. But in general, gastroenterologist is specialist to go to for colonoscopy.