There are some workflows for compilation already available in the “Compile for:” menu. Some use MultiMarkdown and some use Pandoc. Any particular reason for choosing MultiMarkdown for LaTeX instead of Pandoc?
By the way, I know that MultiMarkdown menu option, without any target format specified, allows for any post-processing command. Although its position in the “Compile for:” menu leads one to think it is part of the MultiMarkdown compilations. But I digress… 
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The main factor is that only MultiMarkdown is included in the Scrivener distribution (Pandoc is a bit too large to bundle in the software). So we need a basic compliment of formats out of the box and that is what is provided above the fold. The Pandoc options only appear in the menu if it is installed, and are thus meant to flesh out what MMD doesn’t do. With Pandoc there are so many different output options, if we got into them all it would be hard to know where to stop, and before we knew it the compile type menu would be dominated by most Pandoc stuff. So we just put a few unique options in there, stuff people have asked for over the years like DocBook, and the ePub workflow which would be difficult to automate without Scrivener’s help in assembling metadata and CSS.
As for why the general purpose Markdown + Processing option is called “MultiMarkdown”, that has more to do with the processing pane being added rather late in the beta testing period, after the feature freeze. It would perhaps be a good idea to look into how all of that fits together at some point, but for now that is how it crystallised.