Mac mini major updates tend to only happen when the current processors go out of production.
You may mean when they get superceded with a "newer" version. When Intel introduces new CPUs the old ones don't disappear entirely.
The minis are a lagging product line with little concern about performance.
Previously Mini tracked MacBook/MBP 13" processors. There were/are mobile not lagging product line. Typically there was a gap between Macbook update and Mini update but not huge.
MBP 13" is stuck now. So until the Mini gets a new "dance partner", it has lost traction also.