If we were to add multiple conversion support, we would have to make absolutely sure that cyclical chains weren’t possible. This would restrict the options we have in making items and create a design debt that we just don’t like having.
Chaining multiples together to force all damage into one type and then stacking special DR for it would be cool but it would only work in very very specific ways and for performance sake would probably require us to have one line that it takes. So for example, if you had Fire → Void → Physical, it would work but if you went Fire → Cold → Physical it wouldn’t because let’s say the cold to physical happens before the fire to cold. Then we have this huge amount of edge cases that people need to study to see if the combo they want to use works.
Keeping it as one conversion only makes things much more easy to understand as there is only one extra rule you need to know:
Damage only converts once.