You need water to your east (because you can't fly over people for safety reasons), for a really long way. As in launches from texas have to dodge Florida, Cuba and all the other islands over there.
You need very low population density (because rocket launches and landings require large exclusion zones).
You need to be in the US, because of export restrictions.
You want to be as far south as possible, for efficiency reasons (to get as much kinetic energy from the earths rotation as possible).
You are looking at this launch site, attempting to acquire a lot of land on Florida's coast (not really feasible anymore, it's all occupied), or launching from tiny islands/ocean platforms which will have the exact same fresh water issues.
It is absolutely true that a rocket launched at the equator gets the biggest boost from the Earth's rotation, but stuff isn't put into orbit just for fun, it's often because you want it pass over a specified part of the Earth's surface. To pass over CONUS you need some inclination, and the more inclined an orbit is the less assist you get from the planet's rotation. A polar orbit (inclination 90 degrees) has no assist, and a sun-synchronous orbit is slightly retrograde, where you then want to launch as far north as possible, so you don't have to cancel out as much rotation velocity!
The Soviets put their launch site at 45.9 degrees north not because they're bizarrely stupid, but because they'd like their orbits to pass over Russia. Similarly, no Starlink satellite has had an inclination lower than 42 degrees: https://en.wikipedia.org/wiki/Starlink#Constellation_design_...
Going forward, depending on how much interplanetary traffic there will be, you can imagine Guiana Space Centre (Which is currently hosting JWST, destined for a Lagrange point) will see more traffic, but right now almost everything has some amount of inclination.
The soviet launch site was indeed chosen for other reasons than maximizing payload... in particular I think I recall that it was largely motivated by avoiding launching over China, and being in an empty enough part of the world that it's unlikely to kill to many people (despite launching over land).
If launching from an ocean platform, then fresh water seems pretty easy to bring in via tanker. That's less workable at Boca Chica, or on most tiny islands - where ports for decent-sized tankers are scarce.
To put some numbers on the "north"ness.
Boca chica is at 26 degrees north, which gives you 416 m/s free velocity (1500 km/h) [1]. The southernmost point of Georgia's coast is at 30 degrees or 401 m/s, the southernmost point of South Carolina's coast is at 32 degrees or 393 m/s.
Actually calculating payload to orbit is more work that I don't really want to do, but you can find someone else who put in the work on this stack overflow question [2], just for different latitudes and a different rocket. 28 to 30 degrees is roughly a .3% change in payload capacity in their model.
You're definitely not the first to suggest Peurto Rico, but I think you have the same problem of there not being enough empty land [3].
[1] https://www.google.com/search?q=%28%282+*+pi+*+6378.14+km+%2...
[2] https://space.stackexchange.com/questions/998/how-much-of-an...
[3] https://www.quora.com/Why-don-t-we-build-a-rocket-launch-sit...
https://newsismybusiness.com/11-firms-interested-in-developi...
