Because people want to pay $1 or $2 for a full version of an app, and that may not be enough to justify development.

A windows license is still legacy software model. You dont buy a lifetime windows key, you buy a version key and have to pay again after a major update, although this looks like its currently evolving to a more free to play model. Microsoft has an exponentially wider audience who are mostly captive though, as opposed to LJ who has just had his audience dramatically reduced.

At the end of the day development takes time. Time is money. If LJ cant make a sustainable wage from sync they will have to work elsewhere and sync and its users are the ones who suffer.