I think there are three distinct phases of this game:
Pre-Craglorn
Craglorn
Imperial City
These phases have all been met with their own set of challenges and problems, with a codebase that was growing by the second.
I think that the two original trials were created in the "craglorn" era, and have been coded by so many different people by now it's a wonder that they even work.
Then you have sanctum which was created in the Imperial City phase by seemingly has the same people who created it, working on it now. When you have your own codebase to modify, especially if its modular, it is so much easier than trying to go and "fix" anon functions that you didn't write.
Tl;DR I think it's impossible to "fix" the current code of the old trials efficiently enough to bring them to current standards, so they would have to be brought to code0 and recreated in the same vein as Sanctum, which more than likely is not a valid use of the heavy lifters within the coding department.
Obviously just my observation and opinion, but it checks all the boxes within the "makes me feel good in my tummy" metrics of valid though.
Tl;dr really weigh the fun you have in game vs the business practices you are supporting.