I honestly don't think the length of the albums matter. As bands get more successful, they tour more, and that eats up all of their time. Lots of bands will tour for 2 full years after an album release, and only then start to work on another.
Back in the 70's, bands just didn't casually tour the world like now.
Bands toured like crazy in the 70's. Led Zeppelin II was written and recorded on the road. (technically that was the 60s, tho)
I think you have the causality backwards. Because CDs were a longer format, they enabled bands to take longer tours before having to release another album.
Back in the 70's, bands just didn't casually tour the world like now.