it simply wasn't what anyone asked for or the public had a great need for.
Disney surveyed guests over the years and a park based on animals was always the most popular. In addition to taking business away from BG.
That is why they built it.
Your post is confusing though. You are comparing DAK to a zoo and criticizing them at the same time for a marketing campaign attempting to differentiate themselves from normal zoo?!?
The Nahtazu campaign dropped shortly after the park opened. The reason was not necessarily to say "Hey, we are not a zoo!"
This is part of the story of how Nahtazu came about.
During park previews (resort guests then eventually select public guests and corporate sponsors) there was no initiative or push to get guests to Africa to ride Kilimanjaro in the morning. What was happening was guests were going to other parts of the park (Dinoland, Camp MinnieMickey, etc.) and were not aware or bypassing many of the real animal areas. Africa sat empty every morning until right before lunch. Part of this is that guests were exploring the park and going to the first "land" they came across from the left or right which were non-animal areas. (Coming after the Oasis on Discovery Island).
This caused a knee jerk reaction that made management re-think how to get guests to Africa during the mornings.
They trained all CM's at Main Entrance, Guest Relations, Tickets and Parking to tell guests that the best time to see the animals was in the morning as that is when the are most active. (Not here to debate animal behavior...that is what was told) Information/DAK Preview locations were set up in the resorts to tell Disney guests to go to Africa first to see the animals. It was a pretty big effort.
This caused the crowds to funnel into Africa causing the wait times for Kilimanjaro to quickly go over a 120 minute wait. Complaints rolled into Guest Relations regarding wait times.
There was a huge "uh oh" moment.
After the first few weeks, CM's were told to stop telling guests since telling them was no longer needed. Now they had the problem of Africa being overwhelmed and other areas of the park sitting empty (not completely but enough).
Part of the reason why Nahtazu was launched was to try to re-educate the public that you DO NOT have to see the animals in the morning without telling them that. They figured by focusing on other non-animal aspects of the park through marketing it would undo the "animals-in-the-morning" word of mouth campaign.
Sure, there were plans for a marketing push but Nahtazu was not necessarily the original plan.