The style of beer most widely consumed during Oktoberfest celebrations can be traced to a time before refrigeration was available.
According to the online beer resource BeerAdvocate.com, brewing beer in the summertime was nearly impossible in the days before refrigeration. Hot weather and the risk of bacterial infections complicated brewing in summer, so many brewers would cease brewing beer in spring and begin again in the fall when temperatures dropped.
As a result, the beers available for Oktoberfest celebrations were the Marzen style of beer, or those brewed in the month of March, shortly before brewing ceased for the season. These Marzen beers were kept in cold storage throughout the spring and summer or brewed at a higher gravity so they would keep and be available for consumption during Oktoberfest celebrations.
Such brewing restrictions were no longer necessary after refrigeration became commonplace and made it possible to brew beer year-round, but there's still a strong chance that steins being hoisted during this year's Oktoberfest celebrations will be filled with Marzen-style beers.