Looking at the list of parks that don't have presidents...
-California's Great America is on the way out the door, so it makes no sense to create a new position there at this point in time. I wouldn't be surprised if the park closes after this season, and I would be very surprised if it somehow doesn't close after next season.
-Darien Lake and Frontier City are owned by EPR and will probably join Enchanted sooner rather than later. Given that, it'd be a waste of resources trying to do anything with those properties.
-Dorney Park, Kings Dominion, and New England have all received very recent investment and thus likely don't have a whole lot in the pipeline. Additionally, they likely aren't big enough revenue generators to be prioritized by the chain when it comes to investing in discretionary projects. Thus, adding a new position to oversee the property more directly when that individual will likely have limited ability to actually make substantial changes doesn't make a ton of sense even if they aren't being considered for sale. If the sale rumor is actually true, however, I will say a Herschend grab of these makes a huge amount of sense as it would link together their east coast properties very nicely.
-Discovery Kingdom is in a bit of an odd position, and I have a feeling this is the park the chain is most unsure on at the moment. Everything I've heard points toward the park's ability to pull in visitors from CGA's current market as critical to its future within the Six Flags chain, so I wouldn't be surprised if they're waiting until they know what they're doing here before putting a president in charge. It's also possible they have someone in mind from CGA, but need them to stay in their current role this season before moving over once the season ends.
-Fiesta Texas is the head scratcher in all this, because by all indications this would be one Six Flags would want to keep. Is it possible they simply haven't decided who to appoint? Is it possible they don't feel the park needs a president for whatever reason? Is it possible there's an interested buyer and they're going to consider that offer, thus they don't want to invest in scouting if they might be selling it? I think any of those are very realistic scenarios at the moment.
-Mexico I don't know enough about to speculate on, but if it is being offloaded I'm going to guess it's separate from any other potential deals going on.