Let me explain something. When a question is about the entire franchise, you can't go and answer why ONE of the games is good! Just to clarify, while the Monster Hunter series isn't famous in the West, it's been the number 1 selling game series of all time in Japan ever since the very first MH game! The reason is that, unlike almost every Japanese game, it incorporates element like actual physics and logic. It became know because it was realistic back then and, unfortunately, it's losing that aspect which made it into what it is today. To make you better understand what I'm talking about, think about this. When you start the game, you are hampered by the human (mortal) limitations one has in the real world, but as you advance you can find ways to defy physics. An incredibly strong blast of wind can send you flying or you can find a way to overcome this. That's why it became successful. It was because it introduced overcoming your physical limits in one of the best ways there are. You can never become too strong, just change your strengths/weaknesses. That is the originality it introduced and it still retains until today. Also, another example is the there is no hammerspace, which is in almost every other game! That added extra difficulty and required to strategize and adapt every time you set out on a quest! The last example I'll make is the monsters, they are realistic and based on real world animals or beast, but, most of all, they are the most realistic thing in the entire game. Each one of their body parts has it's own health bar and will break off, get scarred or cut off when it's damaged enough. Just like in real life, if you start pounding a tiger's head, it's tail will never be cut off, but if you try to cut, but not pound, the tail, it will come off! All these have always been the huge differences between the Monster Hunter series and the other Japanese games! The last point I'll make about why the series is so well received is the difficulty! You don't get to choose a difficulty level, the games start off with easy and get progressively harder depending on many variants like the armor and weapon you are using, the monster you are facing, the level of the quest and so on. It can go from being a ridiculously easy game to being a complete nightmare where you are killed by a single tap! However, because of the difficulty, it's not that famous in the west, since people over here rarely are able to play such games!