Interesting, this is my first time consciously thinking about this trend.

Perhaps the needs for read/write speed are bounded (before processor, etc. becomes the limiting factor), while more capacity is only limited by price. Or maybe increasing density of storage inherently means a tradeoff with I/O speed (AFAIK, NAND flash needs to rewrite lots of data just to make a single write? Atom-scale interactions have side effects)

A modern hard drive (36TB @ 280MB/s) can take more than a day. If you treat a bank of tapes as one device this can get even more extreme.