Fallout: New Vegas has been one of the most celebrated games in the franchise, however Bethesda Softworks has largely refused the games' developer, Obsidian Entertainment, the chance to make another one. The reasoning seems to be due to a combination of poor timing, bad luck on Obsidian's part, and Bethesda taking a new direction with their publishing.
Sources/Things I referenced:
Obsidian wants to make another Fallout - https://www.tweaktown.com/news/49876/obisidian-entertainment-make-another-fallout-game/index.html
Obsidian layoffs 2011-2012 - https://www.giantbomb.com/obsidian-entertainment/3010-1520/forums/report-obsidian-hit-with-layoffs-unannounced-proje-539342/
Chris Avellone leaves Obsidian - https://www.rockpapershotgun.com/2015/06/09/chris-avellone-obsidian/
Lone's Video - https://www.youtube.com/watch?v=qUHyIc1OMyU&t=310s
Thumbnail (New Vegas NCR Ranger Armor) - https://www.nexusmods.com/fallout4/mods/9034/?tab=3&selected_game=1151&navtag=https%3A%2F%2Fwww.nexusmods.com%2Ffallout4%2Fajax%2Fmodimages%2F%3Fid%3D9034%26user%3D2%26gid%3D1151&pUp=1