Reeves was written into NCIS in the first place thanks to showrunner Gary Glasberg. Gary had plans for Reeves’ future, but when Gary passed away in 2016, it seems his plans for the character died with him. According to Duane, the producers who were left behind had no idea how to move forward with Reeves’ storyline. It appears that ultimately, they figured the easiest thing to do would be to write him out of the show entirely.
“Clayton was Gary’s brainchild; he created this character,” Duane told TVLine in 2018. “So when Gary passed, I was feeling a bit of panic in people’s faces. I could tell that they were not really sure [what the future held]. I kind of got my bearings together and thought, ‘What’s going to happen now with the character?’ I live in a world called You Never Know, so I’m always prepared for everything.”
Source: https://www.distractify.com/p/why-did-duane-henry-leave-ncis