I thought it was quite clear why it wasn't built for the other platforms just yet.

The other platforms are generally dominated by tech savvy users but in the big scheme of things we make up a very small part of the internet as a whole. I'm sure they just wanted to spend more time focussing on getting a larger number of customers adopting early than spreading resources thinly.

If anything I think it shows a great amount of respect on Google's behalf of its Linux and Mac users, because I dont think they would have wanted to release a buggy piece of crap to their users if they had limited resources on the project.