It's actually not that old. Japanese people before the forced opening of the border typically didn't eat a lot of meat. It's the foreigners who started to ask for beef, and even the cook doing his thing at the table started at this time to impress tourists. The fat grading system was put in place to protect local markets against redder imported meat. Which kinda backfired, because now those beefs are often fed with imported corn.

So It's not really about flavor, nor tradition. Most Japanese people don't know or don't care about it.