In the years before World War I, all sorts of people were improvising armored cars by adding armor and weapons (whether machine guns or light naval canon) to pre-existing trucks and touring cars. Thus, the easiest way to make tanks widely available at the start of the War of 1914 is to populate Europe with lots of tracked vehicles, so many, in fact, that the military folks (and armaments makers) who are making armored cars cannot escape the idea of making "armored tractors."
In our time line, the small number of tracked vehicles in existence were used either for particularly demanding construction work or the working of particularly hard soils for agricultural purposes. Thus, we need such things as increased agricultural exploitation of places like Tunisia and the Central Valley of California, high-value engineering work in places where labor is expensive, or an increase in demand for wood products that led to increased timber cutting in remote areas of the world.
Another possibility is the making of tanks by adding armor to "big wheel" tractors. These were less specialized than tracked vehicles and thus potentially more useful for a variety of agricultural and engineering purposes. One can imagine that demand for these would increase if there was an epidemic that greatly reduced the supply of horses around the world or an epidemic of a disease to which larger horses were particularly prone.