I took Japanese in college, and we were taught that this is because dictionaries are arranged by stroke. For Kanji characters there are tens of thousands of them, so "alphabetical" order is a meaningless concept, making stroke count and order the most meaningful ordering concept. But that then requires stroke order (and direction) something you have to be completely consistent about.
This is a memory from a couple of decades ago, so I could be wrong.
This is a memory from a couple of decades ago, so I could be wrong.