Because the wireless drivers are closed-source "binary blobs," developers cannot simply compile a standard OpenWrt or DD-WRT image and flash it. Without the drivers, the router loses its Wi-Fi capabilities.
Newer macOS versions have dropped support for older protocols (SMB1/AFP) used by Time Capsules. Modern hacks focus on forcing these protocols to stay active or using a Linux box with netatalk to mimic a Time Capsule's behavior. apple time capsule custom firmware