I don't truly know why the devs created multiple parsers. However, I assume that their reasons include the old parser being due for a re-write anyway, plus the opportunity to debug a "replacement"[1] without screwing up all the wiki pages in the meantime. I could be wrong.
[1] NB that I am not saying that Parsoid itself (rather than, for example, some other parser that is partially derived from Parsoid and partially derived from the old parser) will actually replace the old parser. I don't know exactly how they plan to handle that detail.