Perhaps the Shire was too far away, and seeming of little importance until the torture of Gollum, was something that wasn't needed. None of the Istari really gave any thought to Hobbits but Gandalf, so why would Sauron? They hadn't attacked or defied in any way, so why bother?
The Shire was far from most would-be enemies and was also under the protection of the Rangers. The Hobbits of the Shire did see a couple of invasions by Orcs and White Wolves, but even Angmar pretty much ignored them.
Why were the Rangers so keen on protecting an oblivious peasant nation? Did the land have some kind of significance?
Both the Shire and Bree-land were once parts of Arthedain so the Rangers probably felt some responsibility for the safety of their former subjects. Once Gandalf had taken an interest in the Shire, they had one more reason to protect it. And once Gandalf began to suspect that Bilbo's funny magic ring might actually be the One Ring, he asked Aragorn to double the watch on the Shire.
I think there are probably a couple of reasons, the Shire was not particularly close to any of the evil strongholds, so as there was never really any purpose to go there it was never really thought to be worth the effort. Also, I'd probably say a lot of it goes with what Gandalf says in that Sauron saw no threat / had no interest at all in hobbits, so it was probably just a land he assumed would fall in time without any effort.