We added several messages in the update so, when there's any reason why passengers won't show, users would understand why, instead of assuming there's something wrong with GSX. Previously, you wouldn't have any message and passengers won't just show. Now, there's a message, so you know why they don't show.
In your case, the problem is not having stairs, which is of course caused by not having configured the airplane properly for GSX, so there isn't a single staircase available that will fit the airplane, usually because the doors are not set correctly in the GSX Airplane configuration, either because you made a configuration for an unsupported airplane and didn't set the doors correctly, or because you edited the configuration for a supported airplane, and moved the doors to the wrong place.
If, instead, your airplane is really one that cannot use ANY of the available GSX stairs, usually because it's *very* low from ground, like a small GA plane, than that message is just informing there will be no passengers, because of the lack of stairs.
