The next book I picked up after the 80s YA was a 70s MG.
Not an update to be seen. I have a hypothesis about this*
The difference in age group has to be a factor there. There's still something about technology in the hands of younger children that makes parents - and especially grandparents - grimace. It seems that with preteens, there is still enough fight to keep the electronic gadgets away, but by teenagerhood, we've given up, with much moaning and lamenting.
So, leave the older MG books alone, because its fine to have those kids living in an outdated world, (because sometimes it seems like we're trying to raise our kids in a fairyland bubble of what we think the world should be like, instead of how it really is) but update the YA, because teenagers don't or won't read about kids without cell phones and iPods?
* I am basing this hypothesis on exactly one example from each category, which happened to tip off an internal discussion with myself over what parents think of updating older books vs. what authors think vs. what the intended readers think, seeing as the kids who would have read these books when they were current now have children that are in the intended reader age range. I know the perspective of writer and parent, but how about kid reader? Do they care? Will updating or not updating make a book more appealing?