I have been to SeaWorld once in the last 30 years. I go to Universal and Disney annually (this year twice). For me SeaWorld is just not that interesting of a park.
We're just the opposite. SeaWorld is our favorite of all the World Class Orlando parks and the one with the activities we are most interested in repeating. lol
We go to SeaWorld and one of the four Disney parks (We mix it up a bit) once each year (usually just two days per year in an Orlando park on a mini trip). Universal interestingly for us traditionally has been our least favorite -- one day visit in 1989 when it opened, one US/IOA visit in 2008. And now with the new HP and Diagon Alley stuff, we instead of doing a two day mini trip in Orlando are doing three days mid December after DS's finals and before Christmas crowds: one day SeaWorld, one day IOA/Universal, and one day at one of the Disney parks (this year it's back to the Magic Kingdom).
I would think that Diagon Alley and Blackfish are both factors, but not even the only factors. There is certainly a lot of competition in the Orlando market. // Everyone is trying to keep visitors at their parks for as many days as possible, and more onsite rooms getting filled up at both Disney and Universal doesn't help SeaWorld either. Multiday pricing policies too encourage multiple days at parks for the budget minded vacationer (most people) - A two day park to park ticket at Universal is the same price as a one day and a third day not much more than that. (We wouldn't want to spend a second day. But many people would and think three days is the ideal number for a visit. Just visit the Universal forums here and find tons of people doing those three day Universal visits. That's a big change I think.). At Disney four days is the same price as three. And days after three and up to ten are attractively priced. Now for me, one day a year at one of the Disney park is perfect, but that's certainly an expensive cost per visit way to do it. // SeaWorld partnering with different hotels -- offering SeaWorld, Discovery Cove, Busch, and Aquatica packages, etc. has gotten some people to do SeaWorld only trips too. // Orlando competition, though, just looks fierce to me.
Other things in the area that are worth a day too are a day at the beach and a day at the Kennedy Space Center (so much improved than what it used to be and even they are trying to get that second day from visitors with attractive second day pricing and add ons/extras that you couldn't do and fit everything in, in just one day). There is just so much to choose from and do with the typical week vacation. I'm also seeing people I know at least doing shorter trips (especially younger people). They want to take a vacation, but just don't have the disposable income to do a full week.
Does anyone know how Gatorland is doing these days? That's the 1/2 day park that hasn't been on our radar as much recently.
For us personally free cruise offers, Mexico feeling safer in the tourist areas (Puerto Vallarta even hiring more police not that they need it, but to have visitors feel safe and their big Malacon expansion), improvements at National Parks (shuttle systems that help with traffic congestion at poplar times), the new Highline park in New York City and new musicals opening there, etc. are Orlando's biggest competition, and is why we just do two or three days in Orlando annually. (Picking and choosing how to spend that frivolous / fun limited vacation time and limited vacation budget). // Orlando does win for me for the most repeat visits! And SeaWorld wins for us for the Orlando park with the most repeat visits from our family!