The difference is that the iMac Pro and Mac Mini didn’t exist to segment that market back then; all the tiers of professional hardware were rolled into just “the Mac Pro.” Roll those back in and the pricing tiers look more equivalent.
The Mac Mini has been around since 2005...
By 2012, the entry level Mac tower was up to $2499. Since 2013, the cheapest Mac Pro (no PCI/E slots) has been $2999.
The 2019 model is literally more than twice as expensive as the 2013, which in turn was already the previous most-expensive entry-level Mac workstation ever.
PowerMac/MacPro prices have always seemed steep, but this is a new ballgame even for Apple.