Because providing cross-compilation for batteries-included (i.e. fully statically linked) binaries kept being more and more of an issue. The toolchains we have been using before (offered by the RPi foundation) are massively outdated and receive only bug fixes (if at all). All in all, cross-compiling static binaries on Debian wasn't feasible any longer. Almost every update of one of the many libraries we use in FTL caused something else to break, be not recent enough, or ...
In the end, we decided to instead base FTL in Alpine which is designed around this purpose for docker: Provide minimal systems and offer the tools to even create single-binary container images. Updating is now as easy as bumping the base Alpine version. The CI scripts to build binaries have much reduced overall length but - most importantly - there is no hours of fine-tuning required with updates.