Why not use SQL and/or XQuery to query RDF data? Why develop yet another query language?

SPARQL is a query language developed for the RDF data model; queries themselves look and act like RDF. I.e., the queries are independent of the physical representation of the RDF data (the structure of the databases, their representation in an RDF/XML file, etc). If query was done via, for example, XQuery, the application would have to know how that particular RDF data exactly represented as RDF/XML (and RDF/XML is only one of the possible serialization of the RDF data).