Announcing it too early, in a large enough way that the GP notices (not just at the fan/social media level, which makes everything seem like common, general knowledge but it, in actuality, a very small number of people when you take it all into consideration), will also lead people to put off vacations they would have made to either have their vacation when the new resort opens (and deal with the huge opening crowds), thus taking away visits/sales in the current resort, or to miss out on the grand opening madness (potentially putting their trip off by a year or more). Neither is ideal for the current resort (the one that is actually open and making money).
Those who were in and around Orlando at the time IoA was being built, what was the rollout timeline then? Obviously a park was being built, but unless you lived (or traveled there numerous times a year) there, you probably didn't know that. The average out-of-state/country tourist wouldn't know, anyway. Granted, we are in a different time, with social media and such, but I'd think they'd stick to a somewhat similar announcement plan. I remember all the specials about the building of the park, but I seem to recall those coming out in the last 6-12 months before IoA opened, when they were pretty sure it would open on time and ready for visitors. Social media will feed off of the construction picks, the permit filings, the insider info for awhile before UO drops the curtain on the official plans, but those will all be third party reports. UO won't go after the people leaking the info, because it does drum up excitement and interest within a portion of their audience, but they won't go on record confirming/denying those leaks in any way.