Why do airlines seem to prefer 787 over a350?

I have been wondering why people say that the a350 is better than the 787, that the a350 is the better choice, and that the a350 has a better experience. However, I also found out that the 787 is by far outselling the a350. Why do airlines seem to prefer the 787? (To be honest I prefer the 787, being a strong Boeing supporter and fan).

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I think that it's because the B787 is cheaper and can operate on shorter runways and plus it was launched into the market before the A350.

This is quite interesting:

https://twitter.com/byerussell/status/968941766782144515