XNA requires code to be written in C#, though, while LFS is written in C++. Going for multiplatform support, as awesome as it would be, would certainly require a lot of work, and as C# is managed code, the performance on lower-end PCs would suffer if they decided to move to XNA completely.
As for playing on a HD TV, I don´t see where the problem is in doing so with the current version. I am playing it on an HD TV.