Why does syscall compile in NASM 32 bit output while popa does not compile in 64 bit?

Answer

Because the syscall instruction exists (and works) on AMD processors in 32 bit mode too. Historically, Intel used the sysenter instruction, but when AMD came up with the 64 bit extension, they used their own syscall and so when Intel took over the extensions, they also started supporting syscall, but only in 64 bit mode.