Why is the trademark objection review and defense notice not mailed directly to the trademark agency?

I am a trademark agency. You issue the trademark objection review (currently called invalidity review) notice of defense directly to the person being opposed (the original trademark applicant) instead of directly to the agency. I don’t know why? In fact, there are many opponents (original trademark applicants), especially natural persons, who may not receive your defense notice due to various reasons. If you send it directly to the agency where you originally applied, the letter will not be returned by the post office, and you will save trouble. In practice, since the trademark applicant has entrusted an agent, he hopes to hand it over to an agency from beginning to end. It is more convenient for the agency to be fully responsible for the objection defense and even the objection review (invalidity review) defense.

Because in the invalidation declaration, the applicant submitted only the respondent and not the respondent's agency for the review, so It cannot be mailed directly to the original applicant's agency.