Is it simply a matter of only having preserved records of ancient Greek sources but no sources from other lands and cultures? Did Greek science quickly spread into the rest of the contemporary world, such as to Persia and India? If so what was the reaction there? Or was this science already known?
The western world is highly influenced by Greek and roman civilization and ideals
However this does not imply that ancient or classical Chinese or Indian mathematics was inferior.
Katyayana and baudhyayana had sutras that gave versions of pythogoras theorem, some of the Diophantine equations.approximations for square root of two, elements of geometry, etc.
This was in the 7 or 8th century BC.
Later Indian mathematicians (5 bce to 10 ce) included the great aryabhatta, bhaskara 1and 2, brahmagupta, varahamihara, yativrsabha, govindaswami, Mahavira, Sankara, brahmadeva etc.
The place value system, use of zero, negative numbers, beginnings of trigonometry with sine cosine etc owe their debt to aryabhatta and brahmagupta.
These concepts traveled to Europe via Arabia over time.
There were other contributions to astronomy, number theory, algebra etc as well.
The obscurity owed much to the fragmentation of sources, as well as information, translation and interpretation issues, as to the loss of the tradition the most ancient mathematicians worked in
Perhaps someone else can briefly summarize ancient Chinese math contributions; I am not well up on those..