It's because Boeing designed it for 8 across and made their default recommended config with 8 across.
JAL and ANA both fitted out their 787s in that way when they first got them. Then United came along and wanted to cram more seats in, so made the seats narrower with narrower aisles then the design intended.
Since then, every other airline followed United and went with 9 across and ANA later refit theirs to 9 across.
The 787 cabin width is 23cms bigger then the A330. Boeing marketed it as allowing wider seats and a better passenger experience compared to the A330 (which would then, in theory, lead to increased demand for the type and more likelihood of selling all the seats, even if the airline charged extra for a wider seat).
Instead, the airlines saw it as a chance to squeeze in an extra seat per row.