Why were tanks unsuccessful in WWI?

When tanks first started seeing combat in World War I, it was unclear for what purpose they should be used. They were thrown into battle by the British without much planning on what they would do. Additionally, since they were brand new technology, they had a tendency to not function well. They often suffered mechanical failures. Oftentimes the ventilation was poor, and the crews inside would pass out or even die from engine exhaust. The armor could be penetrated by some German guns, and the tanks were very slow moving (only about as fast as a normal man walks!).

However, these early failures led to revised thinking and designing towards tanks. Tanks were built in different styles, some for speed, others to be slow but with powerful guns, and breakthrough tactics began to form- heavy tanks would blow apart enemy lines, and light tanks could rush in to exploit the hole. This would eventually lead to the development of blitzkrieg and other combined arms tactics during the interwar period.