There are a number of reasons why airline manufacturers like tubular fuselages.

Cheapness of manufacture is one. This is a lesson designers learnt shortly after WWII, when Douglas propliners competed with Lockheed products. The gracefully curved Lockheed Constellation, Super Constellation and Starliner were the most beautiful airliners ever built, but the workmanlike tubular fuselages of the DC-6 and DC-7 were much more economic.

An added economy factor is the ability of a straight tubular fuselage to be "stretched" or (less commonly) shortened to create airliners with a different capacity. Boeing 767s, for example, vary in length from 48 to 61 meters, thanks to "plugs" inserted to vary the fuselage length. This allows manufacturers to tune their aircraft to customer requirements, or update them when more powerful engines become available.

Long, tubular fuselages can be provided with several doors to ease loading and unloading and meet the strict evacuation requirements of an airliner. In freight versions, they can be filled efficiently with standard containers, even by sliding in these through nose or tail doors. Dedicated transport designs traditionally have wide, unobstructed tubular fuselages capable of accomodating anything from containers over battle tanks to gloriously expensive "comfort capsules" for generals.

An important factor on modern airliners is that a tubular fuselage with bulkheads at the ends is a strong structure that is relatively easy to pressurise safely. Holding together the wide, cavernous interior of a blended wing-body design is a serious challenge. It may need internal support columns and arches. Also, such a wide internal space may visibly flex in flight, which could cause some mental discomfort to passengers.

Last but not least, having a long fuselage separate from the wing allows the designers to shift the center of gravity to a good location by attaching the wing in the correct place, and provides a convenient attachment point for the tail surfaces. It is possible to tune the stability of such a design; modern airliners finely balance the demands of stability with those of economic efficiency. Achieving stabilitty in a blended wing-body configuration is much more difficult, and an efficient airliner of this type is likely to be aerodynamically unstable, like an F-16, and will have to rely on its computers to stay in the sky.

Also, what about seating? Sitting in the center seat for four hours just fucking sucks, there is no way around it. I suspect this different design would exacerbate that problem.