One way to argue that these sorts of disorders should not be covered is to point out that they are much more difficult to accommodate than physical disabilities.
ADA works well if we are talking about physical disabilities. If a person is wheelchair-bound, a business can build a ramp and give them a different desk and make any number of other changes to the physical environment that will allow the person to work effectively at that particular business. These changes will be helpful to the disabled person and yet will not be too onerous for the business.
By contrast, it is not necessarily possible to accommodate people with emotional disabilities in a way that will be compatible with the needs of a business. It is not, for example, very possible to ensure that a person will be able to have a work environment that will not cause them stress in ways that will reactivate their emotional issues.
Physical disabilities are, relatively speaking, much easier to accommodate than emotional disabilities are. It may not always be possible for a business to change its work environment in ways that will accommodate people with emotional disabilities. Therefore, it is better to leave such disabilities out of ADA.