Gonna pop in here and make a bold assumption bases on common logic:

Final Fantasy X sold well
Final Fantasy X was highly liked among the general playerbase
Final Fantasy X was a breakthrough in gameplaying
Final Fantasy X-2 was "cheaply" put together
Final Fantasy X-2 was not well received by the general playerbase
Final Fantasy X-2 did not sell well

General consumers did not get on board with the -2 idea.

Final Fantasy XIII sold well
Final Fantasy XIII was harshly criticized
Final Fantasy XIII didn't offer any kind of groundbreaking elements to gaming

If FFX did well but its spin-off/sequel did horrible, the general consumer might be looking at the past to see the probabilities of XIII-2, in which XIII was not as good as X, so in that ratio, they must be wondering/afraid of how bad its spin-off/sequel will be.

We could just use a simple scale to represent the idea above. The top game being the most well received, the bottom game being the worst:

BEST
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FFX
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FFXIII
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FFX-2
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FFXIII-2
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WORST

So it is expected to have low release sales, question is, if the game is really good, then the word will spread, and the sales will pick back up in the long run, and SE knows this. Its too early in the game to spell out doom and gloom, but the media thrives on these kinds of yellow journalism so...a grain of salt as a wise person once said.