I don't think Japan spice levels are regional, it's just that actual spicy food is more widely available in cosmopolitan areas. Traditional Japanese food has basically zero spiciness outside of stuff like wasabi, which is a different kind of spicy that kicks through your nose and is gone in moments, or Japanese pepper, which makes your lips tingle but doesn't affect your tongue very much. Their chilis that get used in the spice mixes called ichimi or shichimi are a very low level of spice, and up until the past 10ish years, the hottest stuff you could commonly find was tabasco.

Because of that, you often ran into the same sort of issue in all places without a reputation for spicy, meaning that 98% of the restaurants will make nothing spicy no matter how hot you ask for it, and the other 2% will melt your face when you order the max level.

For sure, Japanese restaurants often have the option to make it spicier (either when ordering or as a condiment you can add yourself). However I was surprised at how many people I knew had very low tolerance and would only order what I'd consider mild (or medium if they felt like challenging themselves).