My guess is since 3E expected PC ability to function to include piles of magic items, destroying them on a regular basis would be a crappy thing to experience over and over. Also, folks just want a faster running game. Very few folks are like Lenefan who think spending any significant time of a session figuring out cascading explosions is a good use of their gaming time.
I do like the idea of removing all +x items form the game so magic items are not part of the game math. I think that would be a step in the right direction for everybody except the treadmill math fans.
I think there's truth to that, although core 3.x did have Mordenkainen's Disjunction, which could permanently wreck all of your magical gear. While I don't think it was ever used at any of the tables I played at, I've heard some horror stories about high level 3.x games that regularly saw Disjunction used.
Personally, I think it's primarily because the rule is too much work for little to no return.
It can consume a ton of time at the table (not only do you need to make a save for every item, for potentially multiple characters, but afterwards you need to recalculate modifiers and such for anything that was destroyed).
Also, many people don't find the idea of having their favorite magic items constantly destroyed to be particularly enjoyable. It's kind of like a character funnel. They can certainly be fun. But most people don't want to play a funnel for every session of an entire campaign. Even systems that use funnels, like DCC, typically have one at the beginning of the campaign and then that's it until it's time to start a new campaign.
Lastly, the results can be a bit silly. The fighter gets hit by a fireball that only deals a small to moderate amount of his hp total, yet somehow his flametongue great sword and magic plate mail become molten slag as a result (and somehow being drenched in that molten steel doesn't cause the fighter any harm)?