The problem is very simple: the A330NEO is not selling well at all. It was supposed to be a cheap alternative to the 787: not quite as good, but much cheaper. The problem is that Boeing has managed to reduce the manufacturing cost of the 787 so much that you can essentially buy a 787-9 for the same price as an A330-900.
The A350 is also suffering from the cheap price of the 787: it is too expensive, so Airbus has to work hard to lower the manufacturing cost...
This is declaration by fiat. Do you have pointers to back this up?
Web searches are much more equivocal. Many pro Boeing but not all. Observations that by type and training and flexibility an a330 fleet with a mix of ranges can suit.
The A350 is also suffering from the cheap price of the 787: it is too expensive, so Airbus has to work hard to lower the manufacturing cost...