First off the car manufacturer doesn't design the trunk space to hold a spare tire. But instead will use a brand of tires on their car called Run-Flat tires. These tires are designed to run up to 150 miles on a flat. In 2013 BMW started to eliminate the need for a spare tire when they started to use the Run-Flat tires on all their models.
BMW is not the only car on the road that doesn't have a spare tire. In the United States, over 30 million vehicles on the road don't have a spare tire. Back in 2006 over 1/3 of new cars produced that year didn't come with a spare tire. The decision was made by car manufacturers to eliminate the need for a spare tire to make cars lighter so they could meet the new fuel efficiency standards.