If they gave people the illusion that they could make WPF cross-platform it would mean a lot of wasted time for both Microsoft and the contributors. So it's pro

If they gave people the illusion that they could make WPF cross-platform it would mean a lot of wasted time for both Microsoft and the contributors. So it's probably better to remove that illusion right away. People can always fork and work on it, Microsoft is just sending the message that they won't do any work to incorporate any of it. Open-sourcing something is not an obligation to keep maintaining something.