Because that would essentially eliminate the 1914 doctrine and they don't want to do that being that it was such a core doctrine for the organization for such a long time.
It would still be there just without the false hope that something has to happen within our lifetime. They already tried that, by the time this new generation teaching fails they will have lost all credibility. I don't think this is adding any urgency to the message, most people don't understand it (because it doesn't make sense). With the first generation teaching there was urgency because the interpretation made sense (if you believe in types and anti-types that is). Many who were taught the generation teaching also didn't realize that the WT had made numerous other failed predictions. I remember being taught that 1975 was never really predicted by the WT society. That a high level apostate came up with the idea. In retrospect they were blaming the whole thing on Ray Franz. Plus, we were about to enter a new century and millennium so the feeling was that something would happen soon. That level of urgency isn't coming back, people don't even talk about Armageddon anymore. The Revelation book has been discontinued and I don't see a replacement coming soon.