Because Giants belt is the BF sword for health. Its a powerful item that is bought outright.
They change how armor/MR was because things like Null mantle/Cloth armor were really bad options after season 3. There was no inherent reason to actually build them except for the boots upgrades and a small number of options.
Also to speak more to how you are comparing it. Before this change there really wasn't an equivalent starting option that built into meaningful items for armor MR. Most of the staple items required Negatrons and Chainvest. Not really a reason to buy the cheaper options. This has changed so that when they come back with a longsword you aren't being screwed with having to wait for a meaningful item. Each stat has something that is a basic item that builds into bigger and stronger things except for the resistances until late.
Health had Ruby crystal which builds into
,
, etc. Longsword builds into things like
,
and
and so on. All the stats had these except armor and MR. Negatrons and chainvest where the items to get and not their cheaper counterparts. This is why they have the change.