The bitter cold of Hoth is no friend to small vehicles, which do not generate enough heat to keep their mechanical parts from freezing and their power generators from failing on a regular basis. High-end speeders mostly avoid these issues, but mass-produced military bikes and many shuttles, starfighters and other transports fail on a regular basis.
For large-scale transport of troops and supplies‒when a few select, customized speeders won't suffice‒two options remain. First, large vehicles such as armored walkers and heavy dropships can be outfitted to survive the cold, so long as they are well maintained. Second, there is the tauntaun‒a bipedal animal that has evolved to survive Hoth's harsh environs and to navigate even during whiteout conditions.