Why Git use the colon (:<branch>) to delete remote branch

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".