As with all other things it is the cost factor that is the deciding factor. Intuit doesn't support older years because it is not cost effective to do so. There are several things they don't support due to lack of need and unfortunately this is one of the items on the chopping block. Windows 7 users had to upgrade 2 years ago so they are not immune. You can use the online program instead if you don't want to spend a fortune getting a new Apple.
The fact that Block supports the older MAC systems a couple years longer than TT doesn't mean they will do it forever ... your system will either need to be upgraded or replaced eventually. The aggressive forced retirement program that MAC employs makes their OS obsolete every three years ... complain to them since they are the ones setting the bar.
Currently block currently supports 10.13 and up only. The 2021 requirement is not yet posted.
https://www.hrblock.com/tax-center/support/software/technical-issues-software/software-system-requir...
TT system requirement is 10.14 and up ... although not posted the 2021 will require 10.15 ...
https://ttlc.intuit.com/community/system-requirements/help/minimum-system-requirements-for-turbotax-...
