It's not so much that we are concluding that there is a correlation here, Relaxo, but that the stimulus is telling us that there is. A correlation just means that two things go together in some way. The correlation in this stimulus is limited to this one poem, but the two things that are, in this case, correlated are 1) it's the only one still publicly performed and 2) it's sung, which is unusual.
From this correlation, the author draws a causal conclusion, and of course it's not well supported because these sorts of conclusions rarely are. While there may never have been others that were sung, it still could be that others were, and that would mean the cause existed without the effect. There could be many other causes, which is a common problem with causal arguments.
One last thing, regarding this statement you made:
The text also says that most other epics were only recited, meaning that there must be others which have been sung
That's not a valid inference here. It is possible that this was the ONLY one that was ever sung! "Most" means more than half, which includes the possibility that all of the others were only spoken. Be careful about how you interpret words like "most" and "some" and "not all"!
Adam M. Tyson
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