One of the largest problems with wildlife habitat is the problem of fragmentation. This is where a block of forest or plains or marsh is split into smaller pieces. This fragmentation can be caused by farmland, housing, or forestry, but most often occurs due to roads and is even a large problem in natural reserves. Roads represent an extremely dangerous crossing for most wildlife, as well as drivers, primarily due to collisions with vehicles. They also cause problems by splitting up wildlife populations (small groups of animals are more vulnerable to being wiped out than large groups), degrading habitat close to roads due to maintenance and pollution, and preventing access to resources on the other side.
Because there wasn't a bridge!
Green bridges have been shown to help lessen the effects of fragmentation and to decrease the number of vehicle collisions with large mammals by an average of 86% close to where the bridge is in place (sec. 4.4). They are currently being used in many areas with a high rate of success. In Florida, several underpasses have distinctly lowered the casualties of florida panthers on highways where they have been installed. Not just for roads, overpasses over a deep irrigation canal in Arizona have decreased the number of deer and other animals from getting stuck and drowning in the canal. Researchers have picked out tons of road kill "hot spots" that could benefit from crossings, but it's still a matter of time and money until they're considered everywhere.