git commit -a -m and push to remote

Question

So, I was having this issue: git push not send changes to remote git repository

I was able to solve it after merging and running this:

$ git pull origin master:master 

$ git commit --amend 

note: had to update message, not necessary

$ git pull 

note: since error: failed to push some refs-working on 2 computers and trying to sync up all)

$ git push
$ git add .
$ git commit -m "commit message"
$ git push

And it finally added to the remote. Hope it helps out anyone, but it made me ask a question:

why didn't this work:

$ git commit -a -m "commit message"
$ git push