If the mains are perfect then you had debris in the rod journal oilers in the crank.
If the mains have the same appearance then you had debris upstream, galleries, etc. look at the cam bearings too.

Rifle brush the crank passages, same with the block passages if you pull the engine down.

I would just Scotch Brite the bearings and wash them with soap and water then with mineral spirits.
I don’t dry any internals with a shop or paper towel. Blow it dry. Then lube it as you assemble.

Of course make sure you have a clean well functioning water and dirt trap/filter on your air line.
You should make sure you have a dedicated air hose after the filter/trap that you use just for engine/trans assembly and that it never gets used without that filter.

Blow the air gun in to a clean, old t shirt and see if trash comes out. Sorry for the long post.