Fans apparently demanded it
Here's a video teasing a PC release of Fable Anniversary you can watch, though I wouldn't -- it's just a bunch of messages, presumably written by Lionhead Studios, asking for this revamped game to come to Steam. The featured petition -- "Make it PC users possible too, to decide once again about Albions destiny [sic]" -- on the other hand, is real. 3,262 people signed it.
For those of you who missed out on the Xbox 360 version, Fable Anniversary is the original Fable (plus its Lost Chapters expansion) with new visuals and an updated control scheme more in line with later games in the series. There are some other enhancements too, like being able to save anywhere, but this is largely the same role-playing game experience.
In his review of Anniversary, which he gave a seven out of ten, Chris said "If you've never played a Fable game before this is a great place to start, but at full price, it's hard to wholly recommend over picking up the original Xbox Lost Chapters edition on the cheap."