It is indeed disappointing and purely on performance and features a MacPro5,1 is clearly as capable as a MacPro6,1 and should be able to run Catalina.
However on an age and business basis the MacPro5,1 should already have long had supported dropped and therefore the fact it can run Mojave was a bonus and this was due to Apple having no choice due to then lack of a successor to the MacPro6,1. Now that the Mac Pro 2019 is launched that no longer applies.
I have seen some mutterings that suggest hacks are possible to run Catalina on a MacPro5,1 and with the minimal difference between a 5,1 and 6,1 this is not unsurprising.
Other than 'planned obsolescence' the security issue referred to by Grant is the most logical reason.
NOTE: This security issue - the fact that the CPU chips in a MacPro5,1 are not being patched to address a new vulnerability is not down to Apple but rather Intel who have chosen or are unable to provide a fix. It is somewhat pointless but who knows, perhaps if there had been a patch Apple might indeed have supported Catalina officially.
Personally my biggest disappointment is the continuing lack of support for Nvidia video cards. :( There is also the fact that in effect Apple are still pushing proprietary AMD cards and whilst the situation is not (yet) as bad as the MacPro6,1 there is a danger this will mean newer better video cards may even if only AMD might still be either slow to be released or not released at all.
(In theory you can fit a 'standard' AMD card in the new Mac Pro but there are no AMD cards with Mac firmware, the fact Apple are using proprietary versions of AMD cards makes it far less likely that AMD will start providing Mac firmware versions of standard cards.)
