the reason why is cross os workflow, which is more and more common. many of us work on multiple os daily, and those of us that do, try our best to find equivalent tools for every platform to maximize productivity. people love alfred so much they don't want to have to do without it on other platforms. that's a very understandable sentiment.
it would be very plausible to build applications that are very similar in spirit and interface and share preference syncing but have different low level implementations. obviously it would have to be separate codebases for macos/linux/windows but nothing that alfred does isn't accomplishable on each platform in its own way, the apps could be designed to functions similarly and have a similar familiar interface. many applications do this, but that would require expanding the development and support teams and maintaining multiple codebases. i get not wanting to expand like that, but have also seen what can happen when another company enters the marketplace with an equivalent product and does. I'm hoping team alfred is the first across this line even if it means collaboration with other small like minded dev shops for the other os implementations. maybe it is just a dream, but it is a nice dream.