DoPo said:
a big part of the reason is because of first impressions. Yes, that's right - you start off the game big and grand, give the players cool stuff to look at and do, and you'll get more people playing the game. If you don't grab their attention, it will falter and go to something else that is shiny.
This.
In Medias Res is a powerful tool because it throws the player into a conflict from the get go and says "YO, player! Pay attention, there's shit going down!". If you don't have in medias res, your story will begin more slowly and a lto of players will think "ugh...This is boring, I'm just learning stuff, when do I get to do something?!"
Also note that In Medias Res can be just trowing you into a conflict, and not always throwing you into the middle of the game. (Actually I find games that throw you into the middle of the plot and then flash back to earlier parts to be kinds of annoying)
To give an example of In Medias Res and why it works, I'll use an RPGmaker game I made last winter where I made use of In Medias Res to start the plot. (Dunno if I'm allowed to link to it. It's called "Illusions of Loyalty" if you wanna look it up.)
I could have begun the game with telling the player about the state of the continent and the political tensions that fill it. But I felt that would bore the player and they'd lose attention rapidly. So instead, I opened up with a full on rebel attack on the capital, and used the action and short scenes to show the player the state of the country (and how they have mechanical death machines). I assumed the player didn't need me to say "This is Sarah Hawke, she is a good friend of the main character". Instead I let the player assume it by looking at how they interacted . I made it so the player will go "wow, this is crazy! What the heck is motivating all of this?!" so they will WANT to know more. Then, after the first big event is over, I hit them with the mission they had to do, gave them a quick primer on the political situation, and then let them explore a bit at their own pace (letting them speak to civilians in the town and other soldiers at the castle if they wanted) before giving them a more in depth look at the politics at the start of the first main "dungeon".
This way, the the exposition that's needed for the story to work is sandwiched by action and exploration, giving the player immediate investment to start with, as well as letting them choose if they want to go talk to everyone and get a better feel for the world.
I think it turned out really nicely in the end. No way I could have done it as well by not using in medias res (at least not without alienating a lot of players)