Demacia and Noxus are sooo 15th century,

while Bilgewater and Inonia are far closer to the 17th Century. That means that Demacia and Noxus desperately need to modernize or be left behind and destroyed by the supior technology of their counterparts.

What do they need to update though? Their weapons. Look at the cast of Champions from Demacia and Noxus. Not a single one uses a gun. Look at Champions from both Bilgewater and Ionia. Gangplank, gun. Miss Fortune, guns. Jhin, gun. Graves, shotgun. While not all these guns would be common in Bilgewater or Ionia (looking at Whisper here) they do have them, and guns are a game changer when it comes to warfare.

While the archers of these respective nations may actually be better and more effective than someone with an old fashioned gun, guns do have one big advantage. Ease of use. Think about it, an archer must train for most of their lives to develop the skills and musculature to be able to actually weld a war bow. A swordsman has similar problems, not just any dandy can pick up a sword and actually use it properly. A gun however takes very little training to use well. You have to know how to load it, keep it in good condition, how to fire it and how to aim it. That's it. That means armies are suddenly much cheaper to make.

Add onto that guns actually have the potential to peirce metal armor and suddenly heavy armor is no longer that useful.

If Demacia and Noxus want to stay as some of the most powerful nations then they need to start Updating their armies with guns. Oh and cannons. Those are pretty good too.