Why did some 19th-century critics describe Georg Philipp Telemann as a superficial composer?

In the 19th century, the reputation of Georg Philipp Telemann suffered as musical tastes shifted toward the new styles of Joseph Haydn and Wolfgang Amadeus Mozart, which effectively eclipsed the older Baroque traditions.

During this period, as the works of Johann Sebastian Bach and George Frideric Handel were being reappraised and elevated, critics began to view Telemann’s vast and prolific output as that of a "superficial scribbler." This negative assessment was largely a consequence of the changing historical perspective that favored the emerging Classical aesthetic over the Baroque styles in which Telemann was a master.1

It is important to note that this 19th-century characterization has since been corrected. Modern scholarship and historical analysis have restored his standing, recognizing the quality, variety, and technical expertise present in his compositions.1