This is mostly because the design figured that the primary way to base the abilities off of Constitution was by letting you deliberately tank your damage capacity to like 1.5 times your level on average, with a small splash at level 1 that gradually diminishes in effect (or even just a flat 5 per level, if your GM is kind enough to give you max HP per HD), because of how you can accept enough burn to effectively tank your Constitution to 4 regardless of what it may have been.
As for Player's Handbook II, I think the best thing they did here was outright print the full spell lists of the beguiler and duskblade in the class description, rather than relegate it to another chapter (as with the hexblade in Complete Warrior, for instance). Just being able to know which spells you can cast while the class description is only a page away is a godsend, especially now that we're in the age of digital collections.