Why Swords And Not Plate

Using bloomery iron to make steel makes exceedingly small amounts of steel.  A couple of pounds for a week's labor on top of about a week of smelting.

Crucible steel makes, on average, five days to get three pounds of steel.  This steel can then be hammered into the shape you want.  Going for more output increases the chances that your heat fails and you are left with something worse than bloomery iron.

You needed about 10 pounds of steel to make a breast plate and that's difficult to get in one go from a crucible furnace of the day.

Risk management.

But in the grand scheme of things...

If you get strict about holding the players to TL3 stuff they have a wealth of choices in armor.

Scale armor is common historically and offers decent protection.  Medium Scale is DR 4 and torso coverage is 28 lb.

Heavy Mail is also common historically and is DR 5* and just 18 lb. for the torso.  You should be wearing something under the armor too.  This is normally something like DR 1* Padded Cloth and will add 6 lb.  24 lb total and an effective DR 6*.

* means it's flexible armor.  That means that hits that it stops can still do some blunt trauma damage...

Plus you can add magic if it's available.  It's almost a no-brainer to cast the first level of Fortify for a +1 DR because it's VERY affordable.