Mostly that MoarVM developers could learn from all of Parrot's successes and mistakes, while not carrying any of the historical baggage. That means cleaner design, less memory footprint, faster execution.
> and why were we stuck at Parrot for so long before giving it up?
Rakudo works on Parrot. Why drop support for it?
Once you frame the question that way, the answer becomes obvious: because there hasn't been big changes to the code generation yet that would make continued parrot support painful. Once we reach that point, the situation will be reconsidered.