Many reasons existed for that rule and this issue has been brought up before. I won't detail every reason as its simply a bit of a waste of time to repeat it all.
A custom is a technique you developed. Its unique and often more times than not, more powerful than cannon techniques. And this is why we allow more powerful techniques to be submited in customs than a equally ranked canon technique normally is. It comes with drawbacks and restrictions. But it also comes with the notion of balance and fairness. A custom is powerful and with power comes responsability. This translates into your own responsability into not teaching your customs light-heartedly. Its not something you should teach to another person on a whim. If you do that, ofc you'll feel the effects as its not something that can be undone. If you make a powerful custom, you should be weary of whom you teach it to. If its someone who will abuse it and cause unbalance in the game, the custom might be nerfed or removed. If its someone who you don't like, that person will be forever able to use your custom, even if you don't like him or her anymore. And if its powerful and everyone knows it, its not special and its not powerful. We saw this with customs that were taught without responsability and ended up becoming an issue.
If we were to remove the limits in teaching a custom to someone, we would also remove the resubmission right (to avoid having 10 people using an old version of a custom and 10 using the new one). We would also seriously tone down the power level of customs even more than what we have. And I would personally remove 30 or 40 customs from the RPG directly. No questions asked. As some customs exist and are special because they are linked to the creator. If they were "spread out", they would not be special and thus no reason to keep them.
The limit stays. If you taught your custom to 6 people already and regret doing so, sorry, but you should have been more cautious to whom you taught them.