But this are not investors. This are companies (or a company), doing business. There are plenty of companies that only operate in one market and if that market is big enough to support them then what's the problem? Expanding to other markets might mean more potential money (or not, since those markets are just as saturated and this with locals). On the other hand there's high risk, definitely much higher expenses, a need for foreign partners and people who know how to operate in those foreign markets, most likely a need to learn a whole new language (or several), for many people (artists + staff) or at least a need for jet more additional staff... So you have artists who are doing well and are relatively mainstream in their own huge domestic market. Sure, they can try to expand to other markets, but besides the points I mentioned previously (high costs, need for additional knowledge/languages, low chance of success - and success wouldn't even mean mainstream, just doing good for a niche act) there's also a fact that this are actualy not products, but people and as such have a limited time of working hours available. Now you can spend those hours on a risky adventure overseas, but why, if you are doing well (and are more than busy) at home. Those artists wouldn't be switching laying on a couch and watching drama for a time working abroad. They would be switching guaranteed profit working time and commitments for something that might work (or not) and would even in best case scenario bring less money than working in local market). Not to mention that by being absent, they could lose the business at home to other acts that would be happy to pick it up.
To put this in simple business terms: covering new markets can be a good idea for a production company, let's say for a home appliance maker. Because if a business is doing well, you can just expand production and broaden your market share. But that doesn't work for, let's say, a high end stylist that works in a righ neigbourhood. Sure, they can try moving around a bit and spend half days in poorer parts of the town where they can either charge less or get less customers. But why, if they are already fully booked and cozy in their own local salon? Of course, they could develop branches (or in this case AKB48 sister groups) and that's one business model but that's another matter (and that only works because 48 groups primarily sell a brand, not 'personalities' like J&A does).
TLDR version: there's only 24 hours in a day and spending them in a proven lucrative home market might not be the worst business choice, because the other option is investing more effort, money and risk just to redistribute a part of those working hours to poorer and smaller markets (or markets where you'd forever be a niche, which is essentialy the same).
So is Coca-Cola (the one I mentioned above) a company doing business?
"... and if that market is big enough to support them then what's the problem" - The problem is you never know if the market will be the same big next month or next year, not to mention the fact that the population in japan is declining and aging.