I assume it's time constraints; they want to ship in 5-6 months. The tweets didn't rule out xwayland in the future (highlighting the possibility of community contributions). If they meet their goals and get Sailfish PR1.0 in a decent enough condition quick enough, maybe they themselves could then move down the todo list and start on the extras like xwayland.

Personally I have no problem with them asking help from the community for things like this, it's worth remembering maemo/Meego/Jolla developers have historically been good Open Source citizens, providing huge contributions to Linux (whilst at Nokia, eg. [1][2]), and using Wayland is a major contribution in itself. So they should have no shame in asking for community contributions, they've made plenty back.