It's because you have to allocate skill points to certain skills, and hitpoints just go into your total. Here's an example - you are a 12th level wizard and make it to 13th. You're lucky enough to have a headband of intellect +6. You gain 3 extra skill points, which you decide to dump into craft: basketweaving, giving you 3 ranks. Later on in the campaign, you realize craft: basketweaving is crap, so you take off the headband, lose the 3 skillpoints, and thus the 3 ranks... then put it back on, regaining the 3 skill points, and putting them in search.
And that's only for the last level. Actually, when you take off the headband at 13th level, you lose 48 skills points (don't forget the bonus at first level).... where did you put those specific 48 skillpoints? Do you remember? Well, now you need to go back and update all those skills... now you put it back on... boom, 48 new skillpoints. You could max out 3 different class skills that way, every single time you take off the headband. Need search, spot and listen today? Ok. After lunch do you need some knowledge: arcana? No problem.
And how *do* you determine class skills? What if you were a fighter 6/wizard 7? When you get those 48 skillpoints, are all wizard skills class skills, or do you need to go back and figure out which levels you took when....
No... it's definitely way better just to not have intelligence be retroactive, and not allow intelligence items to grant skillpoints. Otherwise, it's nearly impossible to keep track of your skills, even with a complicated spreadsheet.
-The Souljourner
