Because GCC maintainers have been saying for years, that they are unwilling to support the weird use case of Debian sparc port, which has 64-bit kernel but 32-bit userspace. I can find discussions about it going back as far as 2009:
This is a weak case. I have 64-bit MIPS hardware running debian and it doesn't seem to matter to anyone that the userspace is 32-bit, though the kernel is 64-bit and I can also run 64-bit MIPS binaries on it.
GCC is also used on Solaris/SPARC which has a 64-bit kernel and 32-bit userland.
I just don't buy the "debian sparc is a [uniquely] weird use case". It's not even unique to Debian, and it's not even unique to SPARC.