Because for under $200 you can have a very reliable gun and 440 rounds of ammo with more power than a 7.62 NATO. Also, you will never need spare parts for a mosin because they don't break. Mosin's are a thing of beauty and until you own one you will not understand why they are so popular.
The history, for one thing. I can't think of any other rifle that has been through as many wars. WWI, Russian Civil War, WWII, Korea, Vietnam, CCCP-Afghanistan, US-Afghanistan.
You forgot a few, Russo-Japanese War, Spanish Civil War, Winter War, Continuation War, etc.
I have not seen the popularity of this rifle like on this forum? What makes this rifle "so popular" To me its ugly as can be. Is it because its cheap, and ammo is cheap? I can think of diffrent rifles that would be cheap to shoot with lots less recoil and more parts available.