It's also important to note that while NASA might make multi-year plans they have to deal with a yearly budget from Congress and Congressional mandates. NASA has a relatively small budget but a lot of different tasks, manned spaceflight is only one of those.

They have to drop whole missions after years of planning and construction because the next year's budget is a billion dollars short. It also pushes out things like SLS development because some scheduled testing or purchase gets pushed to the next fiscal year.