Why does Git use a colon (:<branch>) to delete remote branches?

Why does Git use

git push <remote> :<branch>

as in

git push origin :featureA

to delete the branch featureA from the remote server?

I am interested in why the colon was used as the delete flag.

It's so different from git branch -d <localbranch>.

Why don't we do something like

git branch -d --remote origin <branchname>

or is there a deeper meaning behind the colon symbol that I didn't know?

It is not the meaning of the : per se, but what is present, or rather absent before it.

The refspec format is

<+><source>:<destination>

(optional + for non-fast forward)

So when you do something like git push origin :featureA, you are specifying an empty source ref and basically making the destination "empty" or deleting it.

PS: Note that the refspec of : or nothing doesn't mean push nothing to nothing however. It makes git to push "matching" branches: for every branch that exists on the local side, the remote side is updated if a branch of the same name already exists on the remote side.

The colon isn't a "delete flag". Note that git push and git pull both accept zero or more refspecs as their final argument(s). Now read about refspecs. A colon separates source from destination in a refspec. The command git push origin :foo has an empty source and essentially says "push nothing to branch foo of origin", or, in other words, "make branch foo on origin not exist".