I'm far from an industry insider, but I think it could be LCD panel factories needing to be set up for specific sizes in conjunction with mainstream TV sizes go

I'm far from an industry insider, but I think it could be LCD panel factories needing to be set up for specific sizes in conjunction with mainstream TV sizes going up rapidly over the past ~15 years. That is, the most-updated factories are the ones chasing the TV size trend, and monitors tend to get stuck with the output from the stragglers.