int80h vs syscall

Why does int 80h only work with certain code on x86_64 Linux?
for example:

section .bss
  arg    resq

section .text
  global _start

_start:
  ; code here to fill arg with argv[1]
  mov rax,1
  mov rdi,2
  mov rsi,[arg]
  mov rdx,[arglen]
  syscall

is fine, but

section .bss
  arg    resq

section .text
  global _start

_start:
  ; code here to fill arg with argv[1]
  mov rax,4
  mov rbx,2
  mov rcx,[arg]
  mov rdx,[arglen]
  int 80h
doesn't work with pointers copied from bss, if that's even relevant
rax is filled with 0xfffffffffffffff2, aka -14.