From Ryan on Patreon:
“Why do more rifles not have captive takedown pins? It seems that if someone is developing a rifle from the ground up there are almost no drawbacks to having them, especially if the rifle is going to be widely fielded. Is there a hidden cost?”
Having captive pins does add a few more small parts, machining operations, and assembly complexity to a rifle design. However, more important is that not all materials are conducive to captive pins. There needs to be a way to house the spring and detent pin necessary to create a captive takedown pin, and materials like stamped sheet metal and polymer are not usually well suited to that. Stamped steel has no space, and drilling the necessary holes in this polymer walls creates weak points.