Hello. I’ve searched for it and saw that it was an object of discussion, but I didn’t see any current thread about it (sorry if it exists and I’m just blind) or an official statement of the reason - why set items can’t be improved into legendary?
In my opinion the reasoning behind it is the same as for the unique items, but even more important. When deciding about filling an item slot, there’s this dilemma for the player about if to fill it with a powerful, perfectly crafted rare items with each of the affixes filling the need of the character, or slap a unique item - interesting and strong, but at the cost of losing 2-3 strong affixes that could be there with a rare.
It’s even more emphasized for set items. Players lose more slots and need to fill the important affix slots in stricter circumstances because of that.
So again - is there a good reason for sets being non-upgradable? Unless their whole purpose is to make the build harder to complete, but potentially much more powerful with set bonuses, which is understandable, but needs balancing (ofc that’s a given as it’s a beta)