Earlier quoted context omitted.
The same thing that prevents any enemy from sending a thousand guys to any other sensitive site - men with guns (and intel and air support and armor). Occasionally the cartel and Texas guard do exchange fire.
You think that if a thousand cartel guys showed up at a nuke plant with the intention of melting it down that armor and airstrikes would help matters? I think that would just help them actually.
The Mexican cartels are notably wary of messing with U.S police agents and police/military/border security agencies. There are isolated cases of very foolish sicarios or smaller, more irresponsible organized crime cells playing at threats against U.S border agents or at U.S agents inside Mexico but it's rare and often discouraged brutally by the larger cartels. The simple reason why is they know that with the U.S government, the subversion they regularly practice with pathetically corrupt Mexican authorities works much, much less and goes completely out the window if a U.S police agent dies by their hands. The U.S government strongly underscored this with the cartels by spending decades and God knows how much money diligently pursuing every last person they could find who had any connection to the Kike Camarena murder. The lesson hasn't been forgotten by the bigger cartel bosses, even today.