We'd love to put Rex on Steam! Unfortunately, it costs far too much for us to do so. The plan was always to see how Rex fares here on itch, and if it earns enough, use that towards putting it on Steam. We're glad you like the look of it!
As for 8-9-2017, the above is no longer the case! Thanks to a publishing deal with Pixeljam Games, Rex: Another Island will be headed to Steam. Expect to see the pink dinosaur there later in 2017.