Here's why:
* Complete thought: The sentence expresses a complete thought: "Egyptology is the branch of learning that is concerned with the language and culture of ancient Egypt."
* Subject and verb: It has a subject ("Egyptology") and a verb ("is").
* No conjunction: It does not start with a subordinating conjunction (like "because," "although," "since," etc.).
Subordinate clauses, on the other hand, cannot stand alone as a complete sentence and require an independent clause to make sense.