Things like Kramdown IALs, for example. Apex can handle a lot of (but not all) of Pandoc's special syntax, including cite-proc and bibliography support, but using Pandoc means you don't get features from Kramdown or mmark. Pandoc is über-powerful, but for 80-90% of simple Markdown->HTML cases, I think Apex is going to offer everything Pandoc does plus the benefit of extensions from processors it doesn't handle, and where it does handle them, you have to choose `--from markdown_mmd` or `--from gfm` --- with Apex you can use both mmd and gfm syntax at the same time with one unified parser.

This question is always annoying, but I'll bite:

1. Because Apex processes more flavors of Markdown and seems to have more configurability with regard to Markdown syntaxes.

This is not a knock on pandoc. I love pandoc. I use pandoc every single week day.

2. There is no obligation for you to use it. People can create whatever they want and share it with whomever they want.

The question was asked in good faith, not as a set up for an argument. I'm sorry you feel put upon or need to become defensive about a honest question...

That first sentence I believe. The second not so much.