Setting Environment Variables for execve in shellcode-style Assembly

Question

I am trying to write assembly code (x86 32-bit) to run a shell, I need to pass 3 environment variables to the shell, named xca1, xca2 and xca3, via the stack:

section .text
  global _start
    _start:
      xor eax, eax       

      ; Push "/bin/sh"
      push eax
      push "//sh"
      push "/bin"
      mov ebx, esp
      
      ; Push env vars
      push eax
      push "1234"
      push "="
      push "cccc"
      mov esi, esp

      push eax            
      push "5678"
      push "="
      push "bbbb"
      push esi      
      mov esi, esp 

      push eax            
      push "1234"
      push "="    
      push "aaaa"
      push esi            
      mov esi, esp

      ; Construct argv[]
      push eax            ; Null terminator for argv[1]
      push ebx            ; Pointer to "/bin//sh" for argv[0]
      mov ecx, esp        ; argv[] starts here

      ; Call execve()
      mov al, 0x0b        ; Syscall number for execve
      int 0x80            ; Invoke the system call

The code is assembled, linked and executes on Ubuntu 32-bit with no problem, and it opens a /bin/sh shell, however, when I run the env command, the 3 variables I defined do not appear. I can only see the PWD environment variable.

Any clue why this is happening??