I think that question itself is indicative of Apple's current problem. If I'm shopping for a Dell, I can always just get one with the latest Intel processor, because Dell has no trouble updating their lineup with each Intel iteration. Same with HP, or Lenovo, or really any top name-brand computer manufacturer.
It's not like Intel isn't making advances with each new generation; Broadwell really is better than Haswell, because its 14nm process allows it to run at lower power levels (or alternatively, provide greater performance at a given power level). Skylake makes improvements with a new microarchitecture, including support for the latest low-level hardware features (such as Thunderbolt 3). And Kaby Lake adds a few more tweaks to Skylake, specifically support for higher clock speeds.
And, of course, the integrated Intel Graphics support also continues to improve by leaps and bounds with each new generation.
So I don't think nit-picking about how much improvement a particular SKU would provide is germane here: the question I have is why Apple has chosen not to make any improvements at all.
I should note, Apple does still provide updates for their iMac line -- those are currently running with Skylake chips (two generations past the current Mini). There's no particular reason they couldn't provide these chips for the Mini as well.