I can’t speak to any actual tech limitations since I’m a UX designer and not an engineer*, but I can tell you why we didn’t add manually sorting the hand initially.
Back in preproduction I wanted to see if players were interested in other sort methods so we made a branch with the hand sorted by card type (and even before mana cost). I was pretty surprised, but everyone in our player group really, really hated it. And that was players of all levels. I only kind of hated it :) and the main reason was it got really hard to each turn seeing which cards were the new cards and if you drew multiple cards it was an even bigger pain and Mobile was even...well you get the idea.
That said I don’t think the idea is dead in the water, it just would take time to figure out how to do it the right way. The other tricky part with if we did let players manually sort (as opposed to auto sort by whatever) it would change how scrubbing works on both PC and mobile. Like right now you can scrub back and forth really quickly. Having a manual sort would probably need to have some way to trigger sorting mode and that’s just more work.
The TL;DR is it is not impossible at all, but just not how we want spend our time compared to other stuff. Hope that helps and thanks for the feedback!!!
*I’ve never had one of our engineers say something was impossible, they just get this look on their face where I know it’s going to take a long time!