My guess it was a budget saving move on Dan Curtis' part to just use the various cast members to announce on the show, and stop using the main ABC announcer. I recall some actors saying at the Festival many years ago that Curtis got them to do that as a favor and never paid them for doing that. As for the commercial break announcements, those all ended in 1968, along with the "DS will return in just a moment" spot.
Curtis was notorious for saving every penny he could. Notice that after 1967, there were no longer any extras in the Blue Whale scenes anymore. I find it very strange that the budget for the show was so high in 1966 when they were getting terrible ratings. But then when the show took off in 1968 with higher ratings, the budget kept getting smaller and smaller each year! I have always wondered why ABC allowed that because they had such a huge daytime hit on their hands.