Not strictly true. The only thing that got huge delay was 4.0, which was due to technical difficulties. 5.0 is by and large an extension of 4.0, and arrived roughly on schedule.
PCI-E 6.0 was in research for long because they knew 5.0 would be the end of the way things were working previously.
PCI-E 6.0 was in research for long because they knew 5.0 would be the end of the way things were working previously.