WoW is pretty much the only franchise (other than Overwatch, which is too new) to have not had a major announcement at a non-Blizzcon event. A primary reason people think this Gamescom might finally buck the trend is just due to timing. Waiting until Blizzcon to announce the next expansion would put us 5 months into the final patch of the current expansion, which is significantly longer than it has ever been in the past. In fact, there have been next expansion reveals before the last patch of the current expansion was even live before.
The announcement-beta-release schedule that WoW expansions usually have definitely favors a Gamescom announcement if Blizzard has any hope of getting the next one out before late summer/fall of 2016. A Blizzcon announcement makes a Spring/summer release look really, really unlikely based on their history.
These are all reasons why the expansion announcement should be at Gamescom, though. While I believe it's possible and absolutely hope it is the case, I am not sure how likely I believe it is to actually happen because at this point I honestly wouldn't even be surprised to see them complete their failure with WoD by leaving a year+gap between 6.2 and 7.0