A simple truth that makes the presentation of data on charts as easy as possible. It is based on already developed perceptual patterns of our mind.
If you plot time from top to bottom, it is more likely that people will simply skip this chart as it will take a lot of effort to understand this type of reading the date categories. This model of representation of time is not very familiar to all of us.
Text categories, on the other hand, are the opposite. Their captions are usually longer than just dates. And they are more comfortable to work with vertically — that way there will be enough space to write categories names + we are comfortable reading a lot of text from top to bottom. We formed this habit while reading our beloved books and surfing the websites.