Most times, short term disability plans are funded by your employer. Your employer’s human resources department (whose workers might know quite a bit about the cause of your disability) are the ones who decide whether or not they accept that you are disabled.
After the short term disability period, your claim is usually handed over to a disability insurance company. The people there do not know you, and do not know anything about the cause of your disability. Also, since they see hundreds (if not thousands) of these claims, they do not necessarily see your claim every day, or even every month. Last, but not least, disability insurance companies need specific information from you, to decide whether or not your disability meets the conditions written in the disability policy.
A lot of the problems I see, which start when the employee goes from short-term to long-term disability started because the employer and/or the employee did not understand what specific medical and other information the long-term disability insurer requires so that they can make an informed decision about your disability claim.
Consult a lawyer to see whether or not your long-term disability insurer is correct when it denies your long-term disability claim.