The only common thing I see is that the Macs that were dropped were discontinued before October 2016, which means by October 2021 (when Monterey is most likely to be released) these Macs would have been discontinued for at least 5 years (in Apple's world, <5 years is supported, 5-7 years is vintage, 7+ years is obsolete). In other words, it's like Apple wanted to purge 2014 computers from the list, the same way they seemed to want to purge 2013 computers from Big Sur. For a lot of these computers there isn't a technical reason why they aren't supported. For example, the 2013-2014 MBA and Late 2013 and Mid 2014 13" MBP are very similar internally to the 2014 Mac mini, yet only the Mac mini retains support). The Late 2013 and Mid 2014 15" MBP are very similar to the 2015 15" MBP (especially if both models don't have a discrete GPU). In the first couple of Monterey betas we see that the Nvidia drivers required for models with discrete graphics are still present, which means that (at least as of yet) there isn't a software issue blocking them.
In fact, I'm almost sure that Apple would have removed Big Sur support for the rest of the 2013 Macs if they could. The 2013 MBA and Late 2013 MBP are virtually identical to their 2014 counterparts (in fact, the Apple website at the time didn't even show a change, the only difference was the "Tech Specs" page for each). The model identifier and board ID (2 things the OS checks for to determine compatibility) were identical, meaning they couldn't differentiate between the two even if they wanted to. But the Late 2013 iMacs had no internally similar 2014 models, so bye bye they went. The 2013 Mac Pro was still sold new (and was the latest Mac Pro model) up until December 2019, so it wasn't really feasible to remove this computer from the list (new buyers would have had only one system with slightly less than 2 years to go before full security support would be dropped, and with no major OS updates).
And this wasn't the first time Apple did things like this. Let's take a look at Sierra and High Sierra, computers dropped from Sierra support were all introduced before September 2009 and/or shipped with Leopard. Some of those Macs were perfectly capable of running Sierra, but they weren't officially allowed to. Many of them had the same Core 2 Duo chips, identical Wi-Fi cards, and the GPUs were similar if not identical to the models that came after them. Like a 2009 13" MacBook Pro, a 2009 non-unibody MacBook, and a 2009 unibody MacBook. The all had the similar if not same C2D CPUs that have been shipping since the Late 2008 unibody Macs, the same Nvidia Geforce 9400M GPU, and very similar hardware otherwise internally, only one of those machines would end up supporting Sierra. A similar story with iMacs, MacBook Airs, and Mac minis.
