Because as I already explained (then elaborated on further in my edit), it will be a very long time before most of the things I care the most about will be released on a high definition format, if ever.If you don't mind watching DVDs for a long time to come, why are you bothered about a small loss in resolution? If that resolution mattered so much, then why not get HD Disc with far greater resolution still?
It seems to me that the main point of buying DVDs is to keep them a long time. If one plans to watch a DVD only a few times over a period of several years, it's much cheaper to rent it, especially now that services like Netflix make this much more of a possibility for unusual titles. Criterion should be shooting for the highest possible quality, and catering to those who buy their discs and care the most about technical presentation. You might remember reading my earlier point that if they tried to maximize their profts by pleasing the majority they would not be the company they are. They wouldn't have bothered with preserving OAR, which they stuck with even though it baffled all but a handful of customers in the beginning.That said, I will still be watching many of my disc for quite a while too. And, while it would be good for the slight bit of extra resolution, I don't see Criterion or any other company caring too much. Why should they cater to the minority that are going to keep their DVDs for a long time (without spending money to upgrade), when they can please the majority now (in the hope of increasing profits)?
If you have any evidence that most people plan to either use tube TVs for the rest of their life or replace their whole DVD collection with a new format, I'd be interested. Getting a TV that's better than an old tube is a fairly easy and relatively inexpensive proposition now. Replacing one's Criterion Collection is far more expensive and difficult, the main barrier being actually getting all those films released in a better format. It took Criterion 22 years to release Late Spring. They never released it on laserdisc, and they put out DVDs for about eight years before getting it released on this format. It's far from a sure thing if or when we'll be able to buy it again on a high-definition format.