Why is sle so difficult to diagnose?  

Symptoms vary. Symptom complex can vary from skin rash to weakness and anemia. If the rash is not present or atypical it may not be obvious. Also the blood test is not always positive.
Masquerade. It can mimic a lot of other problems. A rheumatologist can usually clarify the diagnosis. It can masquerade as an infection, heart disease, lung disease, kidney disease, a variety of skin diseases.