I talked about kendo with my colleagues who have no exposure to it.
They were impressed by the non-commercial nature of kendo and said "gee, why I've never heard of such a good activity?".
Simply, there are not enough instructors to make kendo mainstream.
Kendo is chiefly a qualitative activity, and it's time consuming to become even a vaguely competent kendo practitioner to teach other people.
Grade is just a minimum requirement to open a club, but the grade itself is not a reliable indicator of the actual skill. Many organisations issue a grade out of leniency - to keep the organisation viable.
You should be able to tell a good kendo practitioner when they move. Not by their grade.
YouTube & forums are useful but these medium cannot troubleshoot individual issues that are caused by a person's unique set up such as physical and mental dispositions.
The kendo skill has to be transferred from one person to the other manually, which means teacher & students both have to be present in the same space for a considerable length of time.
You can't copy & paste (nor Skype) kendo skill. You & your teacher have to be in one place to make you understand the subtlety of kendo movement, because just 'wacking a target with a stick' isn't enough to call it kendo.
So, perhaps this is the reason why kendo remains a slow-growing activity and would never become mainstream.
But I don't necessarily see it as a bad thing.
Kendo exists because it's a culture that we can share the joy of. Not because it's commercially profitable.
If you practice kendo anywhere in the world, it's a great privilege as well as serious responsibility. The responsibility towards your forebearers also towards your successors.
I think such format is very rare and precious in this day and age.