Well, because we had to invent the massively parallel GPU in between those times. Essentially work that would take an entire supercomputer cluster in the 90s can be done on my desktop with 4 high end GPUs stuck in it.
Now that we are in the range of having the correct hardware the whole "it's taking decades issue" will go away.
GPGPU was definitely part of the success of recent years, but there was also a lot of experimentation and hard work carried out e.g. on CNN designs. Lots of trial and error. That took a lot of time. There have been fundamental changes in the structure and training of NNs that have helped bring the step-change in success.
Now that we are in the range of having the correct hardware the whole "it's taking decades issue" will go away.