Point taken - I was assuming you were just referring to the last spike in visitor numbers at the point the zoo was under threat of closure, in which case you'd obviously have said 1991...
The TFL website is so unbelievably easy to use, really any visitors planning to use the tube should be checking the site anyway before they set off in case there is any engineering work on the tube (there so often is at weekends). Take the bus one time and I guarantee you a nicer journey, unless you like breathing stale air with your face pressed into somebody's armpit for half an hour....
I think the numbers, balanced with the admission price, make the zoo busy enough, as you say. I think great volumes of people are not necessarily desirable in an age when we are becoming aware of the stress caused by the proximity or behaviour of visitors to certain exhibits. Therefore I disagree about the marketing aspect, I think they have got it exactly right. Its telling that I have seen elsewhere on this forum biting criticism of the zoo's misuse of the old terrace cafe building on the Mappins (I was presumed unused, various animal exhibits were suggested), when in fact this is a high-end venue generating revenue through being hired out for events. We have been in recession, ZSL has not even had a mention on here as an organisation under threat, what a contrast to the early 90s.
I think you also need to remember that the aardvarks, hunting dogs, warthogs, hummingbirds, red pandas (which didn't do well), colobus, and mangabeys were all publicised as being attractions within new exhibits when those exhibits opened, and of course the 'Dragons of Komodo', 'Meet the Monkeys', and 'Giants of the Galapagos' were both arguably brand new exhibits with species not previously at the zoo (at least for many years). I would also argue that zoos that have done their market research have understood that, while for you a new attraction might be made more attractive by a previously-unseen species, for the majority of children, an opportunity to 'go in' with the animal in the exhibit would be a much greater draw, regardless of whether the animal has been at the zoo previously.
I think you would be in a small handful of people in this country willing to quote the trades description act because they were disappointed to discover, on their arrival at London zoo, a common-or-garden two-toed sloth when they had been promised a medley of three-toed sloths, common marmosets and spear-nosed bats....I think they are working on the assumption that most visitors would be blown away watching their pair of two-toed sloths make full use of the biome exhibit. Isn't it sad that you could show up at a zoo and not enjoy that because you'd been psyched to see a different species? Where has your wonderment at nature gone?!
People who are good at promotions and marketing don't come with an encyclopedic zoological knowledge, and don't usually have time to acquire one if they are doing the job of increasing revenue for the zoo properly. Yes its irritating seeing every known taxa to humankind investigating a badly-carved pumpkin at this time of year, but zoo enthusiasts are not the target of their marketing! They are appealing to families with children! I'd much rather my city had a good, financially-sound zoo, part-full of meerkats and 'live penguin shows' than a financially-struggling one with mammalian 'gems' in every quiet corner....