Or perhaps since he may be a revolutionist, Dragon told him that he doesn't want Sabo to risk his life so Sabo was reluctant enough to accept that and stayed. Why? Well because the rev. army obviously are still not prepared to show themselves to the world just yet and Sabo understood thatIt's possible that he had no idea of the war until afterwards.
Him not being involved could very well be the same reason the other Strawhats weren't involved: They were too far away and didn't know what was happening.