Who said you couldn't? Epcot-area resort Guests are able/allowed to enter Epcot early and as conveniently as possible for a dining reservation as are Guests using the main entrance.
Please double-check when you check in at BC, but you should be able to walk to the International Gateway, and enter through a designated turnstile where the CMs will likely have a clipboard with reservation names. There's NO reason you or any other BC/YC/BW Guest should need to suffer the inconvenience and expense of having to take a taxi to the front entrance.
If by 'bus to ITT' you mean 'bus to TTC then monorail to Epcot', no, that's a huge waste of time. Plus, a bus from BC would take you directly to MK. You'd need to board the monorail to the TTC, then switch over to the Epcot monorail. You'd probably want to start out at 6:30 AM if you wanted to make your reservation on time.