The first and biggest reason, is the game is also about sharing beers with friends, not just winning. However, let's say you wanted to WIN so you brought some 18% alcohol beer. Now, for starters, those beers are usually more expensive so the likelihood of everyone bringing beers to dominate the game with is somewhat unlikely. Also, in games we've tested with all big beers you tend to ration yourself more because of the high alcohol content. So, you will naturally want a something in your flight to counter balance that...if for no other reason than your own sanity and enjoyment. So, to answer your question, it's something that seems to organically work itself out with the game. Which I think is pretty cool, honestly.
Let's say we have a bottle share and plan on playing a game. (Hypothetically, you wouldn't buy a minimum amount of beer just to play the game...because where is the fun in that?) We have a handful of styles, let's say I brought a couple big beers because I know how this game works.
The SCUM cards (the mutants that are battled in this game and are fueled by the beer brought to the game) have been painstakingly tailored/balanced to slightly punish bigger beers. That's not to say big beers don't do better. I've tested a bajillion scenarios in relation to the beers being played with.
In games where only big beers are being drank: The fights lean toward the more extreme. (just like the beers) So, you either win big or lose big because of the SCUM modifiers. Less PVP situations. Sometimes the game is shorter than other sessions.
In games where no big beers are present: Games will last a little longer, everyone does a little better it seems but there is more competition between players as EXP dwindles. More PVP situations toward the end of a game.
In games where there is a good mix of beers/styles: Most people have 4 completely different beer styles and ABV ranges in front of them. This made for the most unpredictable and fun game sessions. Because the 4 beers you have are randomly chosen each battle by a dice roll this made for the most exciting of sessions. This seemed to happen naturally often as people would always bring a variety of beers because that's what they all wanted to drink anyway.