You forget that the Uchiha were essentially the police in Konoha and would likely be in a situation where the majority of them would be in Konoha. In addition to that, they were currently planning a coup and would need everyone possible in order to be able to have the coup succeed, which means they would try and call everyone and withhold them from doing any more missions once they are back. Not only that, but anyone other Uchiha that may have defected to follow Madara's ways (and essentially becoming missing nin) would likely have been killed by anbu or something and have their eyes retrieved. Most Uchiha also probably aren't capable of using izanagi, either they aren't skilled enough or because it is a forbidden tech among the Uchiha and probably isn't taught.
It makes sense that there weren't any survivors other than Obito, Itachi, and Sasuke. It is possible for there to be survivors, but I'm sure Itachi made sure everyone but his brother was dead.
