Why Medieval Era?

It's easier to write compared to a scifi. In scifi, most of the time you have to bullshit scientific mumbojumbo that makes sense. Most of these authors don't even think about creating a system of magic that is unique and just use the generic magic system that people will recognize. In scifi, you must have an MC that is not an idiot like other isekai MCs because the world is vast enough that most of the time a single man can't make a difference (except Liam) 

Actually, some modern weapons are a lot simpler in concept than old weapons, for example one piece ammunition. Percussion cap with gunpowder in a small container, hit with firing pin. No need for all that powder wad, ramrod, ignition primer, flash pan etc. 

The angels: Your mercy... is, well, being tested. Our God, please thou shan't be in anguish but instead be opportune in their... problems. 

For me, weilding a sword sounds way interesting than a gun fight?!

It's not the answer you want. I have no idea why they keep choosing that particular era. But, I am glad they do.  

But that's backwards thinking or fear of the unknown, particularly the supernatural belief of angels and demons.

What I was explaining was the entropy of arcane knowledge at the inflation of conventional wisdom as civilizations progress. True, they might be certainly connected if we align them together as lines of thought in that particular timeline, but I speak of advancement, not massacres from fear. 

I try using a whole new grammar and jumbled words. Ended up in a mess 

Because melee combat FTW.

On another note, we need more space opera type tropes. Or modern magic style stuff. I believe CN & KN is more popular with these types 

Wasn't armor being used less though, ah but what you're saying does make sense.