It seems like for simplicity's sake, they just made it like the Alpha Quadrant was slap up against the Delta Quadrant. Since they weren't going through the galactic center, they would have had to gone through part of the BQ to get back home to the Alpha Quadrant. And shouldn't the Message in a Bottle where the doctor was transported to that experimental Starfleet ship have been in the Beta Quadrant? It was probably just to avoid confusion by throwing another quadrant in the mix, but it ended up making it kind of confusing if one already knows the galaxy quadrant map.