I think the big thing is that Dragon and Dungeon are magazines designed for a different purpose then say, Wired or Time.
I have a subscription to Wired. I read it normally on the train to and from work, or on the throne. I sometimes pop over to the website if I'm bored at work. Once I'm done with the issue, I put it on a shelf or toss it in the recycling bin. The article was a neat bit of info, some of which is absorbed into my daily life, most of which, however, not so important. It's not my go to source for any real info on the subjects it covers. The entertainment is just in the reading really.
If this magazine went all digital it would loose a lot of value for me. I doubt I would bother with an online subscription.
Dragon on the other hand is much different. While there are SOME articles inside that I think are good purely for the sake of knowledge alone, most of the time when I like an article in Dragon it's because I want to use it in my game. When it goes on my shelf, I'm intending to refference it again int he future.
Having this info in a digital format makes it much easier for me to use. (Just like I now use wikipedia as my encyclopedia, and dictionary.com as my dictionary.)
I think back to when I had my paper copies and how many times I'd hope one of them had an index to all the articles. How many times I'd hope someone posted one once computers entered my world... And how much of an impact the dragon cd-roms had on me. Digital copies of the mag with an index of each article type... A dream come true.
I can see how someone not like me, and who doesn't use computers a lot in his game prep, character prep etc, would be in the oposite boat though. The digital copies might have made life harder.
I can also see that Dungeon, hwoever, is a weird case. I've almost never used Dungeon adventures (or just about any pre-written adventure.) I just can't get comfortable enough with them. Dungeon and pre-written adventures have almost always been a source just of new NPCs, new monsters, new maps and ideas. So for me, it works digitally really well as well.
But... if you DO use and adventure just as written I can see how having to print it out each time would be a big PITA.