I've also wondered this. We saw similar trends in the 2018 elections in Texas. I think it because places like McAllen, Mission, Brownsville, etc. are moreso bedroom communities where people are more religious and therefore more socially conservative. El Paso is a much bigger city and as such more liberal in nature. But I haven't looked into it that deeply.
I think we can exclude El Paso itself from the 2018 races because of the hometown effect Beto had but Lupe Valdez did worse than Wendy Davis in the RGV suggesting that there may be an actual R trend in this region especially because then you can't use turnout as an excuse.
https://www.texastribune.org/2018/10/30/ted-cruz-beto-orourke-texas-rio-grande-valley/There was this article.
