I'm down here in SPI and there are several Mexican restaurants & taquerias here on the island and in Port Isabel.
Most offer a breakfast/brunch and all are open for lunch.
One. One restaurant is open past 5pm.
It's like half the restaurants in the area are closed at dinner time.

Does anyone know why this phenomenon exists?

They start very early with the breakfast taco crowd. I have friends that run a place in Corpus that open at 5:30 AM.

It's out of season for one, a lot of businesses down there shut down for the winter or run limited hours.  Most of the people down that way this time of year are locals or snowbirds.  The blue hair set generally are early to rise and early to bed types.  And part of it is probably covid related, a lot of the old people afraid to go out.  And a lot of businesses, especially restaurants have cratered and shut their doors permanently due to loss of business or have cut back hours because they've had problems getting people to work, money to buy inventory, etc.  Those issues are all over, not just down in south Texas.

I'm originally from McAllen so I'm familiar with Winter Texans and the seasonality thing.

But this is just weird. Pretty much all the other sit down restaurants have a waiting list 30 minutes to an hour plus at 6pm. On a Wednesday night no less.

Just seems like a lot of missed opportunity.

ETA: And there are a *lot* more people here now then when we come down between Spring Break & Memorial Day, usually early/mid May. It's dead then.

Tacos Estillo Jalisco with the beans on the side!