I don't know specifically about Japan, but it sounds like the typical practice of ignoring most cold emails. People are busy. Especially if you try to flood them with information in a first contact it is all to easy to trash a message.
If you are trying to specify a particular thesis topic and will accept no substitutes, then that is also working against you. Most such things require some negotiation and fining a mutually suitable topic.
Make sure you understand the rules about masters admission. Is it necessary to have a designated advisor to apply, as it generally is for doctoral admission? If not, you can just enroll in a program that seems suitable and seek an advisor thereafter. If that is the case, examine the faculty interests at a few places and choose a place that has more than one potential advisor whose interests overlap with yours.
One way to reach out to professors is to do it indirectly. If you have a trusted professor or advisor they can introduce you to others within their circle of contacts. An introductory letter from a colleague, with a short recommendation, is harder to dismiss than a cold email from an unknown person. Since you indicate that your professors don't have such a circle, this is less possible, but not impossible. A letter that introduces themself to someone, saying how they are professionally connected (common interests) along with an introduction of yourself, is still better than a cold mail from you. The better known the letter writer, the better, of course.