They don't have any cars to spare, simple as that.
There was a episode, think it was MASH. The surgeons needed some medical equipment, think it was a incubator or something. They find a supply sergeant has two, so they ask for one. He goes no. They ask, you have two, why cant we have one, your not using them. the supply sergeant goes, if I give you one, then I only have one.
It is not like that. Amtrak REALLY has no spare equipment of certain classes, including all the single-level long-distance equipment.
I know Amtrak does have spare equipment, but I guess they need it for emergencies, changing out a car for a train that is in service. But you have to wonder, would they have too many cars labeled as spare or not?
Amtrak has, if I remember correctly, one spare single-level (Viewliner) sleeping car at NY, one at Chicago, and one at Miami. Zero at Boston, zero at New Orleans. This is actually *less* than the desirable number.
Intermittently a train will go out short a sleeping car and the people in sleepers will be put in coach and compensated, which is really bad from a marketing POV, and needs to be avoided.
The situation is equally bad with the Amfleet IIs used for long-distance coach; there are basically no spares at all, so they are occasionally replaced with tighter-seat-spacing Amfleet Is, which is an unpleasant experience overnight, the sort which drives away customers.
The situation is even worse with the Heritage dining cars, which are occasionally replaced with an Amfleet II cafe car.
Like I said, I know having enough equipment is the issue.
Amtrak really, really does not have enough single-level sleeping cars or long-distance configuration coach cars for the Eastern routes. Period. Adding more cars is *not* an option in the short term; Amtrak manages it on Thanksgiving by arranging to do maintenance at other times of year, but maintenance has to be done at some time of year.
There's a reason Amtrak is currently ordering more single-level sleeping cars. The first one is now in testing -- yay! (See the "Viewliner II" threads.)